Justice. Great article from The Weekly Standard by Walter Berns of Georgetown University and the AEI. Check out his quotations of Camus who got the connection between religious faith and the justness of the death penalty -- if one is a devout believer that is.
I've often told my students that the death penalty per se is not evil and in fact is true justice for those who've been denied life without proper moral justification (i.e. murder and I would go to rape -- especially child rape). The process has been the problem. Advocates who want to end the death penalty want to throw the baby out with the bath water -- so to speak. Fix the process -- keep the death penalty.
h/t: Al Mohler