I thought that the death penalty for rape had been done away with because such a penalty gave the perp the incentive to kill his victim.
We've still got it. And yeah, that's a legit (if rather morbid) reason to get rid of the death penalty for that particular crime.
Surprisingly enough, the feds have never banned the death penalty for child rape. The SCOTUS has left the issue open, and the 5th Circuit hasn't ruled on the issue.