The long-standing argument against death penalty for rapists and child molesters is that such a penalty will give the rapist every incentive — and no disincentives — to murdering the main witness against him.
I’m not saying I entirely agree with that argument, just mentioning that it is out there.
I am familiar with the argument, and I realize you aren’t necessarily supporting it. I support the death penalty for real murder, forcible kidnapping, and forcible rape (which would include child rape in all cases), because it’s biblical, and I trust God in re: his law.