An earlier post in this thread cites a SCOTUS case in which,it appears,that they ruled that the DP is reserved for murderers...not for "lesser" crimes.
You make a fair point about incentivizing killing the victims . However I don’t think that some evil creep raping a child will pause and think “well, l might as well kill this kid they’re going to execute me anyway”. I agree there is doubt this will survive a federal challenge.