What would a Rawlsian decider do? Institute a prison and psychiatric system, and some method of deciding between case 1 (psychiatric imprisonment to try and treat or at least prevent further harm) and case 2 (criminal imprisonment to deter like-minded people and prevent further harm from the killer/rapist). Also set up institutions for detecting and encouraging early treatment of child sex offenders before they moved to murder.

They would not want the death penalty in either case, nor would they want the prison/psychiatric system to be so appalling that they might prefer to be dead.

The Rawlsian would need to weigh the risk of being the raped/murdered child (or their parent) against the risk of being born with psychopathic or paedophile tendencies. If there was genuinely a significant deterrent from the death penalty, then the Rawlsian might accept it. But that looks unlikely in such cases.