Here are the problems I have with the death penalty:
1) As a Christian, I try to make it one of the main goals in my life to convince others to also believe that Jesus Christ died for their sins. That's all they need to do and they are saved (and going to heaven after they die). Unfortunately, they do all the believing in their heart, so you can never really know if someone is saved or not; maybe be extremely sure, but never completely sure. It's scary, but people could lie about it for a lifetime. So, if it's up to me, I want someone to live for as long as possible, so that they have the maximum chance to make the right choice and believe in Him.
2) I don't think that coercive punishment after the fact convinces as well as guilt and morality do. People will be most moral when they don't live under some law that is distant and impersonal, but when they become a law to themselves. When people are truly persuaded--on their own terms--to stop doing something, they really will stop doing it. The law/justice more or less just punishes people, points out what they did wrong, and angers them.
The least we should do is let people live. The better thing to do might be to shelter them from society and try and help them, but at least let them live.