I can respect people like Ed who believe in the rightness of the Church's teachings on the death penalty, but I agree with the prosecutor he quotes who maintains that the death penalty directly prevents worse crimes. OTOH, the argument made by Cass Sunstein and others that the death penalty is a moral imperative for governments goes too far, I think. There are times when the utilitarian argument becomes a tad inhuman, and I think Sunstein has reached that point.
People tend to forget that there is an ancient reason for the death penalty. In medieval times, murders were often avenged by the families of the slain, triggering vendettas that went on for generations. In order to prevent this, monarchs reserved the death penalty to themselves - thus the phrase "capital crimes". This took the possibility of vendetta out of the situation and ensured that relatively impartial justice could be handed down. This is also where the distinction between manslaughter (accidentally caused death) and murder (deliberate killing) comes from - the former merely required payment of weregild to the families of the slain, while the latter called for the headsman's axe. We seem to hve forgotten that one of the functions of government is to provide retribution for the families of the murder victims, perhaps because our cultural elites think we should be better than that after all these years of civilization. Times change, but people do not, and thus we still need a death penalty. People want retribution, and it's better that the state should exact vengeance than that we should leave it up to the families.