To me, the problem with the death penalty is really twofold. First and foremost is reversability. We live in a world of imperfect justice, in which people get away with crimes and people are convicted of crimes they didn't commit. About a hundred people on death row have been exhonorated in the last twenty years. A hundred people could have died for no reason. Thanks to ever-improving forensic techniques, the success rate of proving or disproving crimes is increasing dramatically. But until it is 100%, without fail, I could never even consider supporting the death penalty. This isn't jail. It isn't even caning. It is killing, and the dead just don't come back, regardless of what they taught you in Sunday school. I think that if an executed person is later found innocent, those responsible for sentencing him should be tried for manslaughter.
However, assume that advances in forensic science do reach the 100% certainty level. While I don't see that happening, it's not inconceivable, though certainly decades away. Then my second objection to the death penalty would still apply: the good that would come from killing the convict would not be worth the bad. I don't believe in karma, but I do believe that in order to distinguish the good from the bad there must be a difference between them. Killing should always be a last resort, done only when there is no other choice. There is another choice for murderers, even psychopaths: life in prison without the possibility of parole. Killing them only makes the loved ones of the victims feel better, and I believe that "feeling better" is not sufficient reason to take a life.
I had a bit of a debate with somebody on the VeggieBoards (a vegetarian message board I enjoy) first about President Bush but now about the death penalty. I use the term "debate" loosely since he didn't do much more than attack strawmen, but it inspired me to write about it. Read the thread here, if you like.