well, i beg to disagree, go and ask any victim's family if they would kill the one who killed their relative, if they had the chance, and i'm 100% sure that they will kill in a heartbeat. more to that as a proof is that most families are not satisfied with the judgment of the jury and want to finish it their own way.
I wasn't only speaking about victim's families, but even then I don't think you're going to get 100% on your claim.
What I meant was that as a society many people might agree that a violent murderer should be dead. Obviously there are many people who disagree with this, but there are those who have no problem with the death penalty and think it is necessary. I think most of those people who are in favor of it wouldn't want to execute that person themselves, and are more comfortable relying on a government agency to do it.