The death penalty is not a warning measure but a state's self-defense. Think about it a killer is locked up, not really a big deal to him, 3 meals a day and cable at tax payer's expense, certainly you cannot release him into general populace. The death penalty is a sure measure to ridding a threat to other members of a society
As to rape, rape is not about sex or horniness, it is about the execution of power over a weaker person (even 'date' rape to a limited degree), that is why gang members will hold gang rapes and why prisoners rape each other.