In the final paragraph of “Rightful Convictions,” Joshua Marquis writes:
I can understand how libertarians generally don’t trust the government to get things right and accordingly might be even more leery of the government killing someone. Professor Cass Sunstein proposed in “Is Capital Punishment Morally Required: The Relevance of Life-Life Tradeoffs” that if the series of nonideological studies done in the last decade are right, then having a death penalty spares between 10 and 24 innocent victims of murder. How can we abandon indisputably innocent men, women, and children to homicide?
At the risk of annoying some readers—as I did Milton Friedman after I raised the same point in an earlier critique of capital punishment (“A Killer’s Right to Life,” Liberty Magazine, November 1996)—I would like to know how a statistical study, no matter how sophisticated, can possibly tell us the subjective motives for acts that were never taken and, moreover, how it can do so with the specificity of telling us approximately how many people did not do what they otherwise would have done under different circumstances. Where are these people? And, more importantly, how would we recognize one if we happened across him or her?
Despite my extreme skepticism about statistical correlations in matters of deterrence, I think it is always best, whenever feasible, to accept the strongest case of the opposing side—so I will assume, for the sake of argument, that the claim of Cass Sunstein and Adrian Vermeule is justified. Here is a key passage from their article:
If omissions by the state are often indistinguishable, in principle, from actions by the state, then a wide range of apparent failures to act—in the context not only of criminal and civil law, but of regulatory law as well—should be taken to raise serious moral and legal problems. Those who accept our arguments in favor of the death penalty may or may not welcome the implications for government action in general. In many situations, ranging from environmental quality to highway safety to relief of poverty, our arguments suggest that in light of imaginable empirical findings, government is obliged to provide far more protection than it now does, and that it should not be permitted to hide behind unhelpful distinctions between acts and omissions. 
As Sunstein and Vermeule see the matter, if a government fails to take an action that would lead to desirable outcomes, then its omission makes it as responsible for the undesirable outcomes as it would be if it took a positive action that resulted in undesirable outcomes. Thus if a government fails to provide medical care for someone who would die without such aid, then the government is responsible for that person’s death, just as much as if it had facilitated his death with positive actions.
This is twisted moral reasoning, to say the least. If I and others refuse to contribute to a charity that provides assistance to war refugees, and if some refugees will die without such aid, that does not make us responsible for their deaths. The same reasoning applies to governments.
Likewise, even if we assume that the absence of death penalty statutes will lead to more murders overall, this does not mean that the failure of a government to pass such laws somehow makes it responsible for those additional murders. The individuals who commit the murders are wholly responsible for their actions, period.
In addition, if the deterrent effect of capital laws is their primary justification, then we need to explore some possibilities.
Why not televise executions so they can be viewed by the public at large? This was a major rationale for public executions in earlier centuries, and it makes a great deal of sense. Surely the deterrent effects of capital punishment will be enhanced if people can actually witness the death of a murderer.
Why not make the execution of a criminal long and painful, to the point where it would horrify and thereby deter spectators? Again, this was the point of some earlier methods of execution, such as roasting heretics over a slow fire (it could take 30 minutes or more for the criminal to die), or the process of drawing and disemboweling a criminal, as dramatically and accurately portrayed in the movie Braveheart.
Why not extend the death penalty to serious crimes other than murder, such as rape and child molestation? If capital punishment is an effective deterrent to murder, then it will surely deter some potential rapists and child molesters.
Moreover, if we follow the logic of Sunstein and Vermeule, then a government that fails to impose the death penalty for such crimes is responsible for all the women who get raped and all the children who get molested because no such law exists. If the supposed causal relationship and the moral evaluation that supposedly follows from it apply to the crime of murder, as Sunstein and Vermeule maintain, then exactly the same reasoning would apply to all other crimes. Simply take your pick of crimes that you find especially abhorrent, insist that the death penalty be imposed in such cases, and then blame those who disagree with you for being responsible for the crimes they would allow to happen.
I haven’t even mentioned the classic objection to utilitarian theories of justice, namely that the actual guilt or innocence of the accused would be irrelevant, as long as people believed he was guilty. After all, people will be deterred by what they subjectively believe is true, not by what is true in fact.
My point is a very simple one: The issue of deterrence is nothing more than a sidebar when fundamental matters of justice are at stake. The possible deterrent effect of capital punishment is irrelevant until the justice of capital punishment has been resolved.