I agree with most of the posts, especially the well thought-out and written post of pohnpei397.
I am somewhat surprised to see that, like me, the majority of posts are not opposed to the concept of capital plunishment...only the possibility of error or biased application of the penalty.
One idea that I have which I have not seen in the posts is the very special circumstance in which a lifer without parole kills in prison, merely to elevate his status/reputation as a "bad guy". So long as there is no capital punishment, such a person has no motivation whatsoever to not kill. In this case, therefor, capital punishment is the only way to ensure that this person shall never kill again.
Finally, a word about deterrence. While it is unlikely that many criminals will not ponder capital punishment or the lack thereof in the moment of their crime. Nevertheless, there are many, many death row prisoners that have fought tooth and nail to avoid their death date through endless appeals. So, at least for the condemned, the death penalty is certainly something they are afraid of.