I myself think there is nothing more insane than crime and punishment in our world. Its quite a brutal thing. As for the death penalty its a tough call. One the one hand more evidence might prove that someone is not guilty after all and therefore we might put to death an innocent man. On the other hand, if we are pretty sure that someone has killed someone else and should lose their right to live in society, why should we pay with our tax dollars for that person to have food, clothes and shelter for the rest of his/her life. This is the problem with Ethics, it is hard to come up with clear cut answers. It really can't be concluded with 100% veracity that the death penalty can deter murderers from murdering, especially not with just stats, and there can be increases and decreases at any time because they world contains a level of unpredictability, even more so than maybe the social scientists figure in.
Okay, I got lost. What I am really trying to say is that maybe the death penalty does not deter murderers because they are to a certain extent self-destructive people and they probably don't really care if bad things befall them, at least not enough to stop them from murdering. I think that you would find that people that have killed in rage, have shown that rage previously to their act of murder, were at some level aware of their rage problem, and may have not done what was sufficiently necessary to deal with their issues.
Its a tough call however, on the one hand perhaps we should abolish it with the knowledge that innocent people may be put to death wrongly. On the other why shouldn't we take the risk of putting them to death even though there is a slim chance that they might be innocent, so that people that kill don't get to live after taking the life of an innocent person.
On a whole, I weigh in on the side of abolishing it for this reason. When you kill someone you now have to live with a murderer everyday, yourself and perhaps that is a fate worse than death and if you have no remorse about your crime, than you are not really alive anyway, nor are you truly human.