Instead of executing killers, you send them to a remote island with no guards and no provisions. Good luck living off the land. Not exactly humane, but cheap. If they whack each other, well, that was meant to be their fate. If they don’t, maybe they learned something and deserve to live. Cruel and unusual punishment? No crueler than lethal injection. Am I kidding? Well, let’s just this would probably make a better movie than social experiment. After all, (a) the convicts could build rafts, (b) their buddies on the outside could rent boats, and (c) terrorists might do one-stop shopping there for psychotic recruits.