When Kant argued in favor of capital punishment in the Metaphysics of Morals, he was justifying a penal institution in an idealized civil state. He gave reasons for thinking that murder and rebellion were capital crimes, but he did not claim that murderers and rebels were to be executed regardless of the character of the relevant criminal jurisdiction. According to Kant, not every civil state can rightfully take the life of those who commit murder or try to overthrow government, because not every civil state recognizes the freedom, equality, and independence of citizens in Kant's sense of "freedom," "equality," and independence." Perhaps no civil state in the late 1790s satisfied Kantian principles for a civil state's being properly constituted. Still, it made sense for Kant to ask whether the death penalty could be justified in a state that was properly constituted. In the same way, it makes sense for us to ask whether the death penalty can be justified in a properly constituted state, even if we think that many or most of the civil states in the world that in fact apply the death penalty are guilty of great injustices, or are unjustly constituted.
In earlier work on this topic, (1) I have tried to develop an argument for capital punishment that is a compromise between the one given by Kant and one given by Mill. That is, I have tried to argue that, in principle, the death penalty is justified for the crime of aggravated murder. Mill once identified this sort of crime--brutal murder without extenuating circumstances--as uniquely deserving of the death penalty, but in doing so he gave reasons for thinking that the death penalty is not very severe. He thought that the death penalty only seems to be very severe, and that it is highly desirable for a penalty to appear more severe than it actually is. I reject this whole line of thought. I think it is highly desirable for the true severity of penalties to be widely known, and I find implausible Mill's claim that death is actually a less severe punishment for crime than imprisonment with hard labor for life. The death penalty seems to be severe because it is severe. It is severe because it is irreversible, and it takes away the conditions for all worthwhile human experience. On the other hand, because it is as severe as it is, it should not be applied to murder of all kinds, but only to murder without extenuating circumstances. Kant, therefore, goes too far when he prescribes it for murder in general. But he may be right to recommend it for the severest crime or crimes, as long as requirements of respect compatible with punishment are met. Making a media spectacle of an execution, permitting public ridicule of the condemned person, or describing in ghoulish detail the execution process itself, would violate these requirements.
In arguing for a position midway between Kant and Mill, I follow Kant in taking for granted idealized penal institutions. If people who are not in their right mind are regularly punished in a given jurisdiction, or if sane people who are prosecuted do not start out as equal citizens of a republic whose legislation maximizes the free and harmless pursuit of happiness, or if false confessions are not investigated, then all bets are off and the death penalty, as well as other sorts of punishment, are tainted by the injustice of the relevant civil state. Justice excludes punishments that vary with the social status or race of defendants, or that are applied in some other way that suggests that citizens are not equal before the law. Where such biases exist, even the execution of someone who has committed gratuitous mass murder is morally questionable, because there is too much institutional scope for an innocent person to be executed or for a guilty person to be acquitted because he is well born or has influential friends. The Kantian argument does not support capital punishment for even the worst crimes if it is unjustly legislated or unjustly administered. …