http://www.cnn.com/2009/CRIME/05/12/death.row.exoneration/index.html
Simply put: Because although rare, innocent people sometimes get put to death. I think this is intolerable in a civilized society. Period (as they say). But I concede, this is a TOUGH issue, much like abortion. I would encourage any young person out there, when it comes to issues like these, take your time and don't feel compelled to rush to an end-all, be-all opinion right away -- if you have trouble resolving these kinds of hard issues until you're a bit older (like I was), who cares and that's fine. Better to get it right in the end than to flip-flop around in the meantime.
And here are some of the reasons that make death penalty abrogation such a close call: First, the primary justification proffered by the far left for eliminating the DP has very little merit in my eyes, i.e. that the state has no business or right taking the life of a person, even one convicted of first degree murder. I don't buy into that justification, and I certainly don't agree with it. Second, DP advocates -- leaving aside what I view to be their rather baseless and meritless "deterrence" argument -- make one hell of a compelling case in favor of the DP when they focus on their retribution argument, i.e. that if a person takes another's life in the first degree, then that person DESERVES to die for it. I agree with that basic proposition myself, but I just can't bring myself to get past the fact that on very rare occasions, an innocent person will die. To me, that is simply too high of a price to pay for a punishment with which I would otherwise have little problem.