Surely the defendant's own future danger to society is not the only relevant question. A system of deterrence is a system that attempts to guarantee, ahead of time (it must do this is in order to deter crime) that if someone commits a crime, then he will be punished. The prospect of being punished will deter the would-be criminal. That's what deterrence is. In such a system, a person who goes ahead and commits a crime will be punished. It does not matter whether he himself will commit additional crimes after his initial crime. That's irrelevant to the deterrent mechanism. That person's own "future danger to others" is of no special interest.

You're making a valid distinction; but Snead lumps "deterrence of crimes by others" and "prevention of future crimes by this individual" in together, as being rational, outcome-oriented, and uncompassionate. I'm afraid I did too. I wasn't focused on justice; I was focused on refuting Kant's argument for free will. Your distinction shows that we need to introduce a concept of free will into our reasoning about justice. The original distinction I was going towards, which is still valid, is that we shouldn't merge them, as many conceptions of ethics do.

It is only incompatibilists who believe that moral responsibility and blameworthiness depend on libertarian free will. What we ought to have done in response to science is discard incompatibilism and kept moral responsibility and blameworthiness, rather than retaining incompatiblism and discarding these key concepts which serve essential roles in deterring harms against ourselves and our families and friends.

Yes; that's the theme of one of the follow-on posts I have planned. Both philosophers and the public often think of "morality" not as "doing the right thing", but as "getting God to like you". "Moral responsibility" requires free will to them, because they don't conceive of moral behavior as behavior that's good for society; they conceive of it as behavior that wins bonus points.

I don't know if polytheistic societies tend to have a different conception of morality due to having competition between gods.