Death penalty doesn't make moral sense !
If someone is innocent and we execute him, then we execute an innocent person.
If someone is guilty but genuinely remorseful now, then we execute a reformed man.
If someone is guilty but incapable to be genuinely remorseful because he is a psychopath, then we execute someone who is incapable to care about the wrongness of his crime. He may have intellectually known that killing is wrong, but psychopaths emotionally did not feel it was so.
Imagine yourself in shoes of a psychopath - you are told that if you step on an ant it's wrong and you'll be sentenced to death, but you don't feel "wrong" stepping and killing ants. And in the impulse of a moment decided to squash a few. Then the society hangs you for that and you just don't feel why you'd ever deserve that.
Now substitute ants with people for a psychopath and see why, while they have to be removed from the rest of the society when they commit crime, they cannot be put be morally put to death.