From a reader:

With respect, I disagree. The chief argument against the death penalty is strictly logical. You’re taking the life of someone who (because of incarceration) does not threaten you, but that’s exactly what you’re executing him for. If you can control him to the point where you can strap him on the gurney, you have enough control over him to remove him as a social threat.
We only recognize one justification for an individual to kill: self-defense. Any individual killer, while not under the threat of self-defense, is considered a murderer. But in the case of capital punishment, society can’t reasonably claim self-defense. No individual can commit homicide for retribution, so why can a state? No individual can commit homicide for deterrence, so why can a state? Yes, a state is different than an individual. But how does that difference give the state the right to kill? All attempts to justify the difference inevitably come down to assertions that “the state just does” have the right. But merely asserting that the state has a right doesn’t justify it.

Me: This is all perfectly logical given the assumptions of the reader. But it seems to me the question of whether or not we see the state as an instrument for dispensing justice is very much settled. Indeed, the reader’s objection only makes sense if you take the death penalty as distinct from the state’s other justice functions. But the state has the right to enter your house without permission (assuming probably cause and all that), the state has the right to take your property (subject to all sorts of criteria), levy fines and all sorts of other things that private individuals cannot.

Indeed, I can throw the example of prison back at the reader. The individual has no more right to imprison another citizen against his will than he has to kill him (the citizen does have the right to detain murderers, until the cops come and get him, but not incarcerate them). Yet the reader concedes that the state has every right to imprison murderers, just not to kill them. Well, where does the state get this right to imprison? The long answer requires the invocation of lots of dusty books, historical developments and weighty philosophical arguments. The short answer is it gets it from the same place it gets the right to execute murderers. Or, as the reader puts it, “the state just does.”