Interesting discussion so far, yet I'm only swayed further to opposing the death penalty.
Wrongful convictions have happened and still happen. Even DNA is not fool proof, see the prosecutor's fallacy.
http://www.cs4fn.org/biology/prosecutor.php
http://www.slate.com/articles/technology/technology/2013/06/dna_math_if_police_find_a_genetic_match_that_doesn_t_mean_they_have_the.single.html
Plus it can be planted, contaminated, mixed up or transferred.
http://wrongfulconvictionsblog.org/2013/06/28/how-innocent-mans-dna-was-found-at-killing-scene/
Taking away someone's freedom is enough. Isn't that the greatest value in the USA anyway? Why do some people feel like prison is equivalent to summer camp with video games in your comfy room. Is life in the USA that bad? Even where it seems exactly like that http://www.theguardian.com/society/2013/feb/25/norwegian-prison-inmates-treated-like-people it still works better than what US prisons accomplish.
I'm not saying all prisoners should go there, yet throwing everyone together in a depressing place doesn't work. Do these prisons have any positive effect on crime rate? http://www.imgism.com/5-of-the-most-brutal-prisons/5/
Prison reform aside, the main goal is to get the person out of regular society. Life in prison is good enough for that, no need for the death penalty.