I am totally against death penalty. Some would come and say: when you do something wrong you have to pay for it. When you kill someone, you deserve death too. But I must ask: the guy who places the criminal on the electric chair, or puts all the seringes on the condemned, will he pay for killing human beings? There's something Albert Camus wrote in a book called "La Chute": (forgive the "freestyle" translation) Not all guilty people, all criminals should be condemned or punished, just so that the honest people won't feel innocent. That's the thing to avoid."
It is true though that prisoneers with long sentences are very expensive but I believe there's a better way than killing them. And you have to be even more careful when considering all the possible juridic mistakes. Also, there's a quite eloquent quote from someone I forgot saying: "We can't give death penalty because we don't know what death is. What if it was reward?"
Edit: reviewing my grammar. Please bear with my sometimes wobbly english.