Until recently I've always pretty much favored capital punishment (where did that euphemism come from? It's the death penalty!). But when I looked at why I felt anyone should be put to death, it was always because "he deserved it". I didn't really think about its intended purpose. But aren't punishments created to dissuade undesirable behaviors?
Lately, and especially with Scott Peterson's sentence, I've begun to wonder if the death penalty really serves any useful function other than to make us feel better--"let's stick a needle in the damn baby killer!" But does anyone think Peterson hesitated for a second because he thought he might get the death penalty? I don't think so. Will Peterson's execution make other would-be killers reconsider? Research doesn't consistently bear it out.
So, if a punishment is not a deterrent to crime, then it is merely vengeance. And vengefulness is not an admirable trait in anyone and it's not a healthy reflection of our society.