Many of these people are heavily traumatized. Charles Manson for example was mistreated by his mother and in prison. If there were better child protective services and a more humane prison system in place, he might never have done what he did.

Most people doing evil things are in part doing these evil things as a result of their circumstances. By changing their circumstances through social programs you reduce violence and make everyone happier as a result.

You can argue that blaming violence on circumstances ignores people's own responsibility, but I believe that better social and medical programs are still worth it because they attack the part of the root cause society can influence most easily.

Rule of law and systemic governance will always be more practical to manage this, than dependence on heroic vigilante justice to meet an evil individual with a heroic individual, on the spot and ready to act.

There's a variety of reasons for this, but one of 'em is that in a condition of systemic governance, there's some societal pressure against the kind of sadism you mention. In a world like Sparta, it's hard to tell the heroes from the villains, and anybody might lash out at any time. Even the quotes given, spell out situations where random violence was just a part of everyday, expected life. This causes tensions and grudges.