Actually it doesn’t matter if the rapist is killed for his crime because he is a rapist. Your faux concern for the victim to protect the criminal from justice smells.
It is a reasonable concern. As it is now, the rapist knows he will never be executed if his victim survives. If the penalty is the same whether or not he kills his victim, why would he leave her alive to report the rape, identify him and testify against him? It will always be to his advantage to kill his victims then, always.
Rapists certainly deserve the death penalty, but if it means more murdered victims, it is not worth it.