The Necessarily(?) Moral Argument for/against the Death Penalty
This, I think, is because sufficient constitutional process remedies most, if not all, of the practical issues of the death penalty. To the extent that these practical concerns are with, not the death penalty in an absolute sense, but the risks of inappropriate execution, constitutional process provides a sufficient remedy.
Volokh's argument that the death penalty is a means of silencing dissenters may apply to governments generally, but our system of appeals and habeas review remedies this problem by giving multiple chances to be heard in court, and years before one sentenced to death actually dies. I think constitutional due process, and habeas review that can't be suspended in a single case, may fix this problem.
Volokh notes that even if we think the death penalty is okay in the U.S., "we may want to discourage the death penalty in other countries, such as emerging but fragile democracies or mildly oppressive autocracies."
This assumes that we would somehow fail in our ability to export constitutional process, but succeed in our ability to export banning the death penalty. I don't know whether that's a fair assumption, it seems that the two should be equally accomplishable.
Perhaps the concern should be that discouraging the death penalty internationally is important because most other countries can't afford the amount of process that we can. The issue is, therefore, not that other countries wouldn't adopt constitutional process, but that they can't for financial reasons. Maybe this isn't true either; I know nothing about the amount of process afforded in European countries. In fact, it seems that the EU is set up in a way that may allow more process in that defendants can appeal claims to an extra-national human rights court. If constitutional process is being violated, I'm sure such appeals could smoke out the bad trials.
Volokh's other arguments are largely against the risk of government error. This, I think, is the best "practical" argument against the death penalty. Even one who has no moral problem with executing guilty people has to take issue with executing innocent defendants.
But this too can be remedied with some combination of science and process. Ever seen CSI? This may prove too much; there may be no way to completely abrogate error. It may require a moral tolerance for both the death penalty and a very small risk of unjust punishment.
But what about a system where only those who admitted to murder, after sufficient process, were candidates for execution?
I guess my argument is that at the end of every one of these arguments is a moral choice. I see no way around it. Those who have no moral problem with the death penalty in the abstract may nonetheless take issue with unjust execution, but isn't that also a moral argument?
And in any event, I think that constitutional process can sufficiently contain "just" execution to "appropriate" situations. This requires a trade-off: more process means less people being executed. But all that means is that one has no moral objection with killing guilty murderers, and has to let some people go who were actually guilty because one has a moral objection to killing innocent murderers. But these are moral arguments.