You suggestion is also good for punishment im thinking before the rapist kill let them feel whats like to be rape like. Let them rape by the prisoners. And after that kill it.
The death penalty
If someone has done you wrong, you have to punished them. However, it depends on the range of their wrong. If they harm you or insult you and you believe you can still manage yourself, refrain yourself from making a sin but if they have physically and emotionally ruined such as rape, a bigger punishment must be given to them. It is their entire life that has been ruin. That is something they can not take back. Forever or life imprisonment must be given to them.