So here’s the rationale that is generally used: If you are in a country that utilises the death sentence then the only system that can decide that is the legal system. Vigilante justice, even when morally justified in the immediate, is not a rigorous or systematically moral justice system. Ergo if anyone is in danger of being killed then they must be protected, even if they are a terrible person, as they have not been sentenced to death (or even if they have, that sentence is not to be meted down by just some other random person).

If you are in a country with no death penalty, you as a society believe that no-one should ever be killed as retribution or as an example to others, thus the argument for protecting people from serious harm is obvious.

These same basic arguments apply for corporeal punishment.

Those who are believed to have committed horrific crimes such as those you mentioned will be in extreme danger because their crimes are fairly universally considered reprehensible (because they… You know… Are). The danger is that there is no perfect justice system. Miscarriages of justice do occur and whilst you may believe that actual perpetrators should be killed or maimed in prison, the risk is that innocent people may be subjected to a horrific and irreversible punishment for no crime at all. That is not acceptable to most people within most justice systems.