Hmm...I believe it's natural for people to want an eye for an eye, but there has to be something that separates us from the animals besides thumbs and Texas Hold'em.
I don't believe it's a deterrent, if someone wants to go on a killing spree they already don't care if they die. And while they are on death row, they get to do it in clean jammies with 3 squares a day while their lawyers work on appeals for years.
Their are 2 sides to this paradox, I choose to lean on the side that claims no one has a right to take another's life, no matter what the circumstances. Others lean on the other side, that's their right.