I'm all for the death penalty, as long as you killed someone. If you didn't kill anyone, then I think it's a little unfair.
If you killed someone in self-defense and it's provable that was the case, then not only should the death penalty not apply, but I think the charges against you should be severely lightened (not removed totally). That way it says "We know you had a right to defend yourself and you only could do it one way, but killing is still wrong and so we can't let you off the hook 100%, we don't want people to get the idea that 'hey, I killed someone.. but it was self defense!'".