It makes no sense at all. Morally, they need to die. No other consequence fits. Economically, a bullet is still less than a dollar. What is the cost of maximum security for one young inmate for life? Regarding justice, those who don't receive it often create more injustice. See the recent stories about the two assassinated NY cops. Regarding society, not disposing of your waste poisons you with it. Regarding deterrence, there is no deterring a zealot. But the sure and unflinching death of zealots would certainly deter more normal human beings from picking up their twisted torch.