IMO, permanent solitary confinement should be what rapists, murderers and sundry types of brutality should win them, on the second round, after they do the standard 25 years 12 years parole with good behaviour.
second time they are confirmed to not be human after being locked up and given time to ponder their situation. no need to kill them though. vengeance does not bring back their victims, and the satisfaction is empty.
it's enough to just exclude them permanently from society.
oh and in case of apocalypse, they can starve to death.