I’m still catching up on the Corner, Jonah, so I may have missed a relevant part of the it-doesn’t-work thread. Regarding the death penalty I think that you are right to say that deterrence cannot “justify” the death penalty. I’d amend that slightly though.
I’d say that deterrence does not suffice to justify the death penalty but can be a component of a justification. It does not suffice because (for example) executing an innocent man widely thought to be guilty would be wrong even if it had a strong deterrent effect. For those of us who believe that the death penalty is immoral (at least in modern societies), deterrence isn’t enough to overcome that problem–any more than we could cite the undoubtedly strong deterrent effect of torturing the guilty to justify doing so. But if those of us who so believe are wrong–if the death penalty is not presumptively immoral–then any deterrent effect it has would be among the things you could rightly consider in deciding whether to maintain and apply it.