That and "governments make mistakes" are the two arguments you can use pretty effectively against the death penalty on people who are not morally opposed to it.
Personally, I have no problem with the state putting a bullet in the head of a murder, but the thought of a wrong verdict and some of the batsh_t crazy things courts have done leaves me fearful of any capital punishment.
I am particularly unamused by some of the things prosecutors have done that have lead to false convictions and how few remedies and punishments for corrupt prosecutors.
Plus, in the sad cold-blooded way of limited budgets, I can find no justification for spending the extra money.
In a perfect world there would be no capital crimes committed. In a near perfect world courts would be 100% right. We live in neither and should act like it.
e.g. (semi-randomly picked links on the topic)
http://www.hg.org/article.asp?id=31614
http://www.forbes.com/sites/kellyphillipserb/2014/05/01/cons...
hmm maybe I'm misconstruing your comment though...