@georgeob1,
georgeob1 wrote:There is little doubt that swift, inescapable and not obviously unjust punishment does indeed have a deterrent effect on human behavior.
While I wouldn't disagree with this statement, it's based on a number of assumptions that by no means can be taken for granted. I would say that the "effect on human behavior" can only be taken as a valid statement as long as we are talking about average, rationally thinking humans. In that regard, I would question the deterrent effect any kind of punishment has on someone who has obviously left the realm rational thought quite a while ago.
Furthermore, it assumes that the punishment will be swift, inescapable and not obviously unjust. I would argue that none of that is true for the death penalty: as applied, it is not swift (with sometimes decades of distance between the crime and punishment), it is not inescapable (not every murderer is caught, and a prospective killer might even be completely convinced that he, unlike others, will most certainly get away with it), and even in a completely transparent justice system, there will always remain questions concerning specific cases.