Spina, J.
After a jury trial, the defendant, George F. McLaughlin, was found guilty of involuntary manslaughter and arson of a dwelling house and not guilty by reason of insanity of two charges of murder in the first degree. The only issue at trial was whether the defendant was criminally responsible at the time of his acts. The trial judge committed McLaughlin to Bridgewater State Hospital (Bridgewater) and stayed the execution of sentence on his convictions until after his release from Bridgewater. Having granted McLaughlin’s application for direct appellate review, we affirm his convictions. We conclude, however, that the judge erred in staying execution of sentence until after McLaughlin’s release from commitment.
McLaughlin was not convicted of murder in the first degree. We therefore do not review his appeal pursuant to G. L. c. 278, § 33E. See Commonwealth v. Cullen, 395 Mass. 225, 228 (1985). A verdict of not guilty by reason of insanity is not a “conviction” within the meaning of § 33E. Cf. Commonwealth v. Doane, 428 Mass. 631, 631-632 (1999). McLaughlin does not ask us for § 33E review in any event.
1. McLaughlin does not dispute that, shortly after killing Albert Myers and David Wallace at the nursing home where he and they worked, he set a fire in his room that resulted in the death of Cedric Clarke, who resided in the room above McLaughlin’s. McLaughlin’s sole defense at trial was that he was insane at the time he committed these acts. Once the issue of insanity was raised, the Commonwealth was required to prove beyond a reasonable doubt that McLaughlin was sane (criminally [508] responsible) when he committed the crimes with which he was charged. See Commonwealth v. Keita, 429 Mass. 843, 849 (1999). To meet this burden, the Commonwealth was required to show beyond a reasonable doubt either that McLaughlin had no mental disease or defect or that he had the substantial capacity both to appreciate the wrongfulness of his conduct and to conform his conduct to the requirements of the law. See Commonwealth v. Goudreau, 422 Mass. 731, 735 (1996), citing Commonwealth v. McHoul, 352 Mass. 544, 546-547 (1967). McLaughlin argues that the jury’s verdict of not guilty by reason of insanity on the murder in the first degree charges requires the same verdict on the other charges. He contends that the judge erred in denying his motion for a required finding of not guilty by reason of insanity. To answer this contention, we must decide whether any rational jury could have come to the four verdicts. See Commonwealth v. Cullen, supra at 228; Commonwealth v. Lunde, 390 Mass. 42, 47 (1983).
“[T]he rule is well established in criminal cases that mere inconsistency in verdicts . . . will not render the verdict of guilty erroneous even though such inconsistency may have indicated the possibility of compromise on the part of the jury.” Commonwealth v. Sherry, 386 Mass. 682, 698 (1982), quoting Commonwealth v. Scott, 355 Mass. 471, 475 (1969). Accordingly, “a defendant may be found not guilty by reason of insanity or impaired mental condition as to one charge, and guilty as to other charges, even where all arise out of the same criminal episode.” People v. Bielecki, 964 P.2d 598, 605 (Colo. Ct. App. 1998). See Commonwealth v. Chandler, 29 Mass. App. Ct. 571, 580-581 (1990).
We are not persuaded that the verdicts in this case were inconsistent. McLaughlin told police that he undertook to kill Myers and Wallace and one or more residents of his apartment building because he believed they were harming him with high-frequency beams. He carried out the plan against his first two victims, but decided after he got home that he would not kill anyone else. In his own words, he “just couldn’t go through with it”; he had “just had enough.” Instead, he elected to bum down his apartment. Before leaving the building, he told a fellow tenant about the fire. McLaughlin thought that the building’s fire alarm would alert all the tenants to the fire. In light of this evidence, the jury could have found that McLaughlin’s mental condition abated by the time he reached his home so that he [509] was sane, according to the McHoul definition of sanity, when he set fire to his apartment. The jury could also have found that they were not satisfied to the same degree of certainty that McLaughlin was sane when he killed Myers and Wallace. These conclusions are not logically contradictory. Cf. Hotema v. United States, 186 U.S. 413, 421 (1902); Commonwealth v. Rogers, 7 Met. 500, 502 (1844) (charge to jury by Chief Justice Shaw); State v. Snow, 513 A.2d 274, 277-278 (Me. 1986). The inferences to be drawn from the evidence were for the jury to make. See Commonwealth v. Kappler, 416 Mass. 574, 579 & n.4 (1993).1
McLaughlin attempts to derail this reasoning by positing that the jury could have reached the two guilty verdicts only by relying on the “presumption of sanity.”2 Relying on the “presumption,” he contends, was impermissible in this case because the two not guilty by reason of insanity verdicts necessarily imply that the “presumption” does not apply to McLaughlin. We disagree. The verdicts of not guilty by reason of insanity did not require the jury to disregard the “presumption.” The jury could have relied on the “presumption” to reach guilty verdicts on some charges while concluding that it did not allow a finding of criminal responsibility beyond a reasonable doubt as to other charges. See Commonwealth v. Kostka, 370 Mass. 516, 528-529 (1976) (in Massachusetts, “presumption of sanity” does not disappear from case after evidence of insanity has been introduced).
[510]*5102. McLaughlin also argues that the prosecutor prejudiced the case by presenting inflammatory, irrelevant testimony and by rousing the jury in his closing argument. The testimony was of a bystander who witnessed rescuers’ attempts to retrieve Clarke from McLaughlin’s apartment building using a ladder. Before the rescuers could reach him, Clarke jumped or fell from the window of his apartment, landing on the ground. The witness testified that he put his arm around Clarke and cried at him to hold on to life, and that Clarke held on desperately to his arm for a minute, then relaxed his grip. This evidence was not inflammatory. The prosecutor did not mention it in his closing argument.
The closing argument was nonetheless improper. Near the end of the argument, the prosecutor ridiculed the trial’s emphasis on the state of mind of the defendant. Shortly afterward, the judge sustained an objection to the prosecutor’s utterances. The prosecutor, however, continued in the same vein. He exhorted the jury to consider the rights of the victims and to do justice for them. He then told the jury that “in the end the truth is that this case is all about whether or not he’s guilty. It’s not whether he’s insane or not. The issue for you is whether he’s guilty or not guilty. That’s the point.” (Emphasis added.) He concluded that “[t]he point is, is that he’s guilty because you say so. And I’m asking you to say so and hold him accountable and hold him responsible” (emphasis added).
The prosecutor erred egregiously in making these statements. He should not have asked the jury to consider the rights of the victims, see Commonwealth v. Barros, 425 Mass. 572, 581 (1997), or to do justice for them, see Commonwealth v. Drumgold, 423 Mass. 230, 253 (1996). Nor he should not have exhorted the jury to ignore their responsibility under the law, as he did when he told the jury to declare McLaughlin “guilty because you say so” and to ignore the question of his mental condition. Contrary to the prosecutor’s suggestion, the case was “all about” McLaughlin’s mental condition. The suggestion therefore could have had the effect only of encouraging the jury to find the defendant guilty even if the evidence did not prove criminal responsibility. See Commonwealth v. Santiago, 425 Mass. 491, 501 (1997), S.C., All Mass. 298 and 428 Mass. 39, cert. denied, 525 U.S. 1003 (1998). The prosecutor should have considered his obligation to argue the Commonwealth’s case “in a way that states the evidence clearly and fairly and inspires [511] confidence that the verdict was reached based on the evidence rather than sympathy.” Id. at 494. He did not do so.
We evaluate the effect of prosecutors’ inflammatory remarks to the jury according to “whether the improper statements . . . ‘constituted prejudicial error’ ” (citation omitted). Commonwealth v. Santiago, supra at 500.
The cumulative effect of all the errors in the context of the entire argument . . . and the case as a whole is considered in making this determination. . . . The following factors are considered: whether defense counsel seasonably objected to the arguments at trial; . . . whether the judge’s instructions mitigated the error . . . ; whether the errors in the arguments went to the heart of the issues at trial or concerned collateral matters . . . ; whether the jury would be able to sort out the excessive claims made by the prosecutor . . . ; and whether the Commonwealth’s case was so overwhelming that the errors did not prejudice the defendant. . . . (Citations omitted.)
Id.3 Three of the factors weigh in favor of the defendant. McLaughlin’s counsel made timely objection to the prosecutor’s remarks. Some of the most salient remarks were in fact directed to encouraging the jury to ignore the central issue of the trial. The Commonwealth’s case as to the defendant’s state of mind cannot be said to have been overwhelming.
Consideration of the second and fourth Santiago factors, however, allows us to conclude that the remarks were not prejudicial. First, the judge properly instructed the jurors to set aside feelings of sympathy and of bias or prejudice. See Commonwealth v. Gordon, 422 Mass. 816, 831 (1996). Although she failed to instruct the jury that a closing argument is not evidence, the effects of this omission were mitigated when the judge told the jurors that “your collective memory of the facts . . . will govern.” More importantly, the judge properly instructed the jurors at length about the question of criminal responsibility. These instructions countered the prosecutor’s attempt to distract the jury’s attention from this crucial issue. Second, the verdicts [512] show that the jury were able to distinguish wheat from chaff. We ordinarily assume that jurors are reasonably sophisticated and capable of sorting out hyperbole and speculation. See Commonwealth v. Marquetty, 416 Mass. 445, 451 (1993). See also Commonwealth v. Rock, 429 Mass. 609, 616 (1999), quoting Commonwealth v. Kozec, 399 Mass. 514, 517 (1987). The verdicts bear out this assumption. By acquitting McLaughlin by reason of insanity of the two most serious charges against him, the jury eloquently demonstrated that they had done their duty. See Commonwealth v. Rock, supra at 616; Commonwealth v. Gordon, supra at 831.
3. McLaughlin argues that the judge improperly instructed the jury on the elements of arson of a dwelling house, G. L. c. 266, § 1. The judge correctly told the jury that the Commonwealth must prove that the defendant wilfully and maliciously set fire to his apartment. See Commonwealth v. Cooper, 264 Mass. 378, 380 (1928). She then said that “malice used in this context of arson is not the same as malice used in a murder charge. It’s used in the sense of improper, selfish motive. But there need not be any specific [sz'c] to kill or any specific intent to do grievous bodily harm, or indeed, any specific intent to do an act that will result in death.” The judge also said that “[t]he use of both willful and malicious in the statute does not indicate that malice has a meaning different from its ordinary meaning. Malice ... in the crime of arson need not be overtly expressed. It may be inferred, jurors, from the willful act of burning without legal justification.” Because no objection was made to this instruction at trial, we review the instruction for whether any errors created a substantial risk of a miscarriage of justice. See, e.g., Commonwealth v. Ely, 388 Mass. 69, 74 (1983).
The trial judge in this case deviated in a number of respects from the instruction on the malice element of arson approved in Commonwealth v. Niziolek, 380 Mass. 513 (1980): “malice ‘characterizes all acts done with an evil disposition, a wrong and unlawful motive or purpose; the wilful doing of an injurious act without lawful excuse.’ ” Id. at 527, quoting Commonwealth v. York, 9 Met. 93, 104 (1845).4 First, she invited the jury to “infer[]” malice without having defined the term. See [513] Commonwealth v. Niziolek, supra. Second, she implied that the meaning of the malice element of arson is identical to the ordinary meaning of the word “malice.” See Commonwealth v. Lamothe, 343 Mass. 417, 419 (1961), quoting State v. Pisano, 107 Conn. 630, 632 (1928); Commonwealth v. York, supra. Third, she described malice as “used in the sense of improper, selfish motive,” rather than “wrong and unlawful motive or purpose.” Fourth, she used the phrase “legal justification” instead of “legal [or lawful] excuse.”5
The malice instruction was erroneous. It nonetheless created no substantial risk of a miscarriage of justice. The defendant concedes on appeal that he acted wilfully (i.e., intentionally, see Commonwealth v. Luna, 418 Mass. 749, 753 [1994], quoting Commonwealth v. Welansky, 316 Mass. 383, 397 [1944]) in setting the fire and that his act was unlawful. Because wilfulness and the commission of an unlawful act are two of the three components of malice in arson, the only component that the defendant has not conceded is the lack of justification or excuse for his actions.6 The only claim of excuse or justification that [514] McLaughlin raised at trial related to his mental condition.7 No other issue was disputed at trial. See Commonwealth v. Gabbidon, 398 Mass. 1, 5 (1986); Commonwealth v. Ely, supra at 74; Commonwealth v. Glenn, 23 Mass. App. Ct. 440, 444 (1987). The judge’s instructions made clear the Commonwealth’s burden as to criminal responsibility. By declaring McLaughlin guilty of arson, the jury necessarily rejected his only defense of excuse or justification.8
4. The judge sentenced McLaughlin to from nineteen to twenty years in prison on the involuntary manslaughter conviction and to from eight to ten years on the arson conviction, the sentences to run consecutively. She stayed execution of the sentences until the date of his release from his civil commitment. McLaughlin argues that the stay exceeded the judge’s authority and violated his constitutional rights. The Commonwealth argues that the judge had the discretion to defer the sentences until McLaughlin’s release from commitment. Without reaching any constitutional questions, we hold that the stay was unwarranted.
As the Commonwealth concedes, McLaughlin is entitled to credit toward his sentences for the two years he spent confined in jail and in Bridgewater State Hospital between late 1994, [515] when the crimes occurred, and early 1997, when the sentences were imposed. See Stearns, petitioner, 343 Mass. 53 (1961); G. L. c. 127, § 129B; G. L. c. 279, § 33A. See also Commonwealth v. Milton, 427 Mass. 18, 23-24 (1998). Similarly, “[i]f he [had been] committed to the hospital after he had started to serve his sentence his time there spent would be credited on his sentence.” Stearns, petitioner, supra at 56. See G. L. c. 123, § 18 (c); G. L. c. 123, § 15 (e). Cf. G. L. c. 127, § 119 (same for confinements in medical facilities); Commonwealth v. Godfrey, 420 Mass. 561, 567-568 (1995) (awarding sentencing credit for time served at treatment center for sexually dangerous persons). The Commonwealth’s argument therefore could apply only to persons who are committed to a facility for the care and treatment of the mentally ill between trial and execution of sentence, not to persons committed before trial or while serving a sentence. To allow the denial of credit because of this fortuity would create an anomaly raising questions of basic fairness.
If McLaughlin is eventually cured of mental illness, then whether his sentences have been stayed will determine the date of his eventual release. Persons who are committed for mental illness while serving their sentences and are cured before the end of the term of their sentences, however, would assuredly be entitled to leave prison by the end of such term. It seems odd at minimum that McLaughlin but not these others should face the possibility of additional decades in prison after his release from Bridgewater. A statutory scheme that permitted outcomes of this sort would raise difficult questions of due process and equal protection. See Foucha v. Louisiana, 504 U.S. 71 (1992); Jones v. United States, 463 U.S. 354, 369 & n.18 (1983) (persons found not guilty by reason of insanity on criminal charges may not be punished for acts with which they were charged); Commonwealth v. LeBlanc, 370 Mass. 217, 221 (1976); White v. Pearlman, 42 F.2d 788, 789 (10th Cir. 1930).
“Although fairness is the concept underlying due process, our decisions in this area have not rested on constitutional requirements. Where no statute controls, we have been establishing guiding principles, case by case.” Chalifoux v. Commissioner of Correction, 375 Mass. 424, 428 (1978). See Lewis v. Commonwealth, 329 Mass. 445, 448 (1952). We do not favor “overly technical” readings of relevant statutes; rather, we read them “against the backdrop of fair treatment of the prisoner.” Commonwealth v. Grant, 366 Mass. 272, 275 (1974). See Com[516] monwealth v. Beauchamp, 413 Mass. 60, 62-63 (1992), and cases cited. See also Manning v. Superintendent, Mass. Correctional Inst., Norfolk, 372 Mass. 387, 392 (1977).
The parties point us to no statute or rule, nor can we find any, authorizing or forbidding judges to stay the execution of a defendant’s sentence until his release from commitment for mental illness. Stay of execution of sentence is allowed in certain other circumstances, but these are not relevant to this case.
Rule 31 (a) of the Massachusetts Rules of Criminal Procedure, 378 Mass. 902 (1979), offers us some guidance. That provision “confers discretionary power to stay the execution of sentence pending appeal.” Commonwealth v. Allen, 378 Mass. 489, 496 (1979).9 Rule 31 is directly derived from the version of G. L. c. 279, § 4, that was in effect prior to an amendment that coincided with the enactment of the rules. See Reporters’ Notes to Mass R. Crim. P. 31, Mass. Ann. Laws, Rules of Criminal Procedure at 313 (Lexis 1997).
In Commonwealth v. Hayes, 170 Mass. 16 (1897), the court held that St. 1895, c. 469, an early version of G. L. c. 279, § 4, granted trial judges only limited power to stay execution of sentence. The defendant had moved for stay of execution of sentence following an unsuccessful appeal. The ground for the motion was the defendant’s poor health. Writing for the court, then Justice Holmes said that the trial judge lacked the authority to grant a stay in such circumstances. First, the defendant’s motion did not fall within what were then the terms of the statute. See id. at 16-17. Second, even if the motion could be regarded as a motion for a stay to be granted on authority other than that accorded by the statute, “[t]he term in which the sentence was pronounced was past, and the power of the court to modify it was at an end.” Id. at 17. In 1897, criminal terms were abolished by statute, rendering irrelevant this basis for the holding in Hayes. See Commonwealth v. O’Brien, 175 Mass. 37, 39 (1899). See also District Attorney for the N. Dist. v. Superior Court, 342 Mass. 119, 122 (1961). Left untouched, however, was the [517] court’s statement that “[a]n ordinary sentence of fine or imprisonment imports that it is to be carried into execution forthwith.” Commonwealth v. Hayes, supra at 17. The court quoted from a precursor of G. L. c. 279, § 34, which provided (as § 34 now provides) that when a defendant is sentenced to imprisonment or a fine, the clerk of the court shall “as soon as may be” (for which the current version of § 34 substitutes “forthwith”) deliver to the sheriff or to another court officer a court transcript authorizing the officer to execute the sentence, and that the officer “shall execute it accordingly.” See Weiner v. Wentworth, 181 Mass. 15, 17 (1902). In Commonwealth v. O’Brien, supra at 39, the court said that Hayes “implies that, under the statute, the power of the court after exceptions are overruled to vacate the order staying the sentence, and to order the sentence executed, does not extend so far as to permit a further stay of the sentence on independent grounds not affecting the legality or propriety of the conviction.”10
When the Legislature codified what is now G. L. c. 279, § 4, within the Revised Laws of 1902, no substantive changes were made to the statute. Compare St. 1895, c. 469, with R.L. 220, § 3 (1902). Subsequent amendments to what is now G. L. c. 279, § 4, including those attendant on the promulgation of Mass. R. Crim. P. 31, have largely left unchanged the authority of judges to grant stays.11 That is a strong indication that trial judges lack authority to stay execution of sentence “on independent grounds not affecting the legality or propriety of the conviction.” Commonwealth v. O’Brien, supra. To the extent that the former G. L. c. 279, § 4, was at all ambiguous, the rule that penal enactments are to be construed strictly in favor of the defendant, see, e.g., Commonwealth v. Keniston, 5 Pick. 420 [518] (1827), would have counseled against construing the statute to permit execution of sentence to be stayed in order to extend the period of a defendant’s confinement. The same considerations would lead us to construe rule 31 (a) not to authorize stay of execution of sentence for this purpose. But we need not resort to any canon of interpretation in order to arrive at this conclusion. The language of the rule is plain on its face. The rule does not authorize a judge to stay execution of a penal sentence when an appeal is not pending.12
It might be argued that the stay in this case was valid because it was an exercise of a trial judge’s inherent power to stay execution of sentence after it is pronounced. Other jurisdictions are in disagreement as to the reach of this power. See King v. Commonwealth, 246 Mass. 57, 58-59 (1923), citing, inter alia, Ex parte U.S., petitioner, 242 U.S. 27, 48 n.1 (1916). A few jurisdictions appear to have approved of the practice of delaying execution of a sentence of incarceration until after the defendant’s release from commitment for mental illness. See, e.g., Talley v. Beavers, 141 Ga. 110, 112 (1913). Cf. State v. Brown, 12 Ohio St. 3d 147, 151 (1984). The English common law, however, appears to have allowed stays of execution of sentence only in a few classes of cases, all of them cases in which the stay was a true reprieve, rather than a means of extending the duration of the defendant’s confinement. See Ex parte U.S., petitioner, supra at 42-45 (quoting Hale and Blackstone). This evidence counsels against an expansive reading of judicial powers that would allow stays of execution of sentence for punitive purposes. See Crocker v. Justices of the Superior Court, 208 Mass. 162, 166 (1911), quoting Commonwealth v. Knowlton, 2 Mass. 530, 534 (1807).
Our own cases are largely silent on the question of the inherent power of trial judges to stay execution of sentence. Our decision in Commonwealth v. Hayes, supra, to the extent that it speaks at all of the inherent power of courts to issue stays, indicates that this .power is limited. Other than this, very little has been said. See Commonwealth v. Glines, 40 Mass. App. Ct. 95, 97 n.2 (1996) (“Ordinarily, a judge would employ a stay [519] only to allow a convicted person to arrange his or her affairs, or . . . pending determination of an appeal”); Commonwealth v. DeMarco, 387 Mass. 481, 482 (1982) (noting that trial judge stayed execution of sentence for one week to enable defendant to see his parents).13
We have found only one reported case in Massachusetts in which the trial judge stayed execution of sentence during a defendant’s commitment for mental illness. In Commonwealth v. Foley, 17 Mass. App. Ct. 238 (1983), overruled on other grounds, Commonwealth v. Amirault, 415 Mass. 112, 117 n.9 (1993), the defendant was committed for six months to Bridge-water State Hospital after being convicted of rape of a child and assault. The trial judge stayed execution of the defendant’s sentences until the end of the commitment to Bridgewater. The judge indicated that any time spent at Bridgewater after the expiration of the six months would be counted against the defendant’s sentences. See id. at 240-241, 245. The Appeals Court rejected the defendant’s claim that he should be given credit for the initial six months spent in Bridgewater. The court said that “the period of commitment, when added to the length of sentence, did not exceed the maximum sentence which the judge could legally have imposed.” Id. at 245. It could be inferred, though it is arguable, that the Appeals Court thought that the trial judge included the six months at Bridgewater in his calculation of the proper sentence for the defendant. We think in any case that a trial judge should not use his or her power to stay execution of sentence in order to lengthen the defendant’s confinement beyond the period announced by the judge in imposing sentence. To the extent that Foley says otherwise, it is overruled.14
We need not delineate in detail the contours of trial judges’ [520] inherent power to stay execution of sentence. We may say, however, that normally this power should be exercised only with the consent of the defendant and for short periods of time. See Commonwealth v. Glines, supra at 97 n.2. See also United States v. Melody, 863 F.2d 499, 501-502 (7th Cir. 1988) (rejecting defendant’s challenge to stay of execution of sentence pending her husband’s release from prison where purpose of stay was to ensure that defendant’s children had one parent at home at all times and where defendant had not objected to stay at time of sentencing). The basic rule is clear. “Sentences are to be executed forthwith unless suspended or stayed for the exceptional reasons permitted by law.” Mariano v. Judge of Dist. Court of Cent. Berkshire, 243 Mass. 90, 92 (1922), citing, inter alia, G. L. c. 279, § 34; Commonwealth v. Hayes, supra; Weiner v. Wentworth, 181 Mass. 15 (1902). No such exceptional reasons appear from the record. McLaughlin’s sentences therefore must be deemed to have begun running when the trial judge imposed sentence, with credit for the time he was held awaiting trial. When viewed in light of the statutory scheme for awarding credit to prisoners for time spent confined in institutions for the mentally ill, this disposition conforms with “considerations of fairness and a proper sense of justice” and avoids the potential for violation of due process and equal protection principles. Chalifoux v. Commissioner of Correction, 375 Mass. 424, 427 (1978). See Zullo, petitioner, 420 Mass. 872, 876 (1995). Cf. Commonwealth v. Hayes, supra at 17.
The judgments of conviction are affirmed. The stay of execution of sentence is vacated. The defendant’s first sentence shall be deemed to have commenced on January 24, 1997, the date when the trial judge initially imposed sentence.
So ordered.