Here’s my tentative thinking on the matter: We normally, and rightly, tend to tailor sentencing to motivation, even when we think the conduct is rightly treated as criminal. If you’re killing for money, that’s very, very bad: first-degree murder. If you’re killing in a moment of anger, that’s very bad: usually second-degree murder. If you’re killing a dying relative who begs you to ease them out of his misery, the law formally treats that as murder — but my guess is that, when there’s sentencing discretion, the judge or jury (in jury sentencing states) will impose a lighter sentence than if the motivation had been different.
Likewise, if you’re killing in understandable revenge, whether because you’ve been attacked by the victim, because your family member has been attacked, or because you’ve been badly betrayed, that’s still bad, but less bad: voluntary manslaughter. Maybe if we had to do it from scratch in this era of sentencing guidelines, we’d just have one crime (intentional homicide) and various sentencing factors related to motivation. Indeed, the federal Sentencing Guidelines, for instance, specifically call for considering provocation in deciding the punishment for battery:
If the victim’s wrongful conduct contributed significantly to provoking the offense behavior, the court may reduce the sentence below the guideline range to reflect the nature and circumstances of the offense [based on factors such as] … [t]he proportionality and reasonableness of the defendant’s response to the victim’s provocation….[T]his provision usually would not be relevant in the context of non-violent offenses. There may, however, be unusual circumstances in which substantial victim misconduct would warrant a reduced penalty in the case of a non-violent offense. For example, an extended course of provocation and harassment might lead a defendant to steal or destroy property in retaliation.
My guess is that such consideration of provocation is indeed routine in state prosecutions, too, whether state guidelines specifically say so or whether this is just the actual practice of sentencing judges or juries. It’s just that in the pre-sentencing guidelines era, where murder would generally lead to the death penalty, the law dealt with provocation and other aspects of motivation by having different substantive crimes.
This also explains the three approaches to what counts as “adequate provocation” to trigger the voluntary manslaughter rule (setting aside the factual question of whether this particular person was provoked). Some states leave this to the discretion of the jury, just as historically many jurisdictions have at times had regimes in which the judge had broad discretion in sentencing. Some states, like California, leave the jury with broad discretion but cabin it in some measure, concluding, for instance, that “a voluntary manslaughter instruction is not warranted where the act that allegedly provoked the killing was no more than taunting words, a technical battery, or slight touching.” See People v. Guttierez (Cal. 2009). And the laws in some states, like some modern sentencing regimes, try to constrain decisionmaker discretion quite sharply, identifying only a limited set of circumstances that, as a matter of law, can constitute reasonable provocation — for instance, “[t]he only categories of provocation recognized by this court are substantial physical injury or substantial physical assault, mutual quarrel or combat, illegal arrest, and adultery with the offender’s spouse.” See People v. Garcia (Ill. 1995) (emphasis added).
Now, of course, one can debate how much discretion should be left to the judge or jury on this sentencing question, whether the judge or jury should be exercising such discretion and whether particular forms of provocation (e.g., adultery or a sexual partner’s deception about his or her biological sex) should be seen as adequate. But the general principle that the level of punishment should be linked to the motivation for the crime — even when the conduct is indubitably criminal — strikes me as quite reasonable.