I'm with Doug on his "life is precious" caveats. I'm no pacifist and I'll always defend the right of a nation, or a people, or a family, or an individual, to defend themselves when attacked, even if that means killing your attacker. There's plenty of conditionality attached to that, I think. If a gunman is on a killing spree in a school, college, supermarket or in the street, then shooting to kill them seems justified. But the death penalty is different to all those cases because it's done in ritualistic cold blood. All the other arguments apply of course, but, for me, that is the clincher. Putting people to death in that way dehumanises us. Makes us no better, possibly even worse, than the person we're executing.