Back to the issue of punishment: what is justice? What gives us the ability to decide what that is, and what gives us the confidence to end a person's life because we think they deserve it, based purely on our interpretation of justice?
How can anyone be confident that ending someone's life is the right thing to do? Philosophically, how the fuck can anyone know that?!? Non-religious people should look at logic and reason, the objective truths (it does very little to help, and 1 in 25 are executed wrongly) and come to that conclusion, while any religion worth it's salt should value mercy over revenge, and it's my belief that religious people should still look to logic and reason, especially when it comes to the law, which is the law for everyone, not just them.
But again, philosophically, how can anyone be sure that "justice" dictates one's life should be ended? (I also haven't seen a good argument in favour of it, outside of the ever-subjective "justice." Certainly not an argument better than the anti-death penalty ones.)