I don't think it's justified to kill anybody unless they present a clear immediate threat to someone. Like, if somebody is lunging at you with a knife intending to kill you, it's okay to shoot him in the face.

What should we do with violent criminals? Well, if we don't want to just propagate more violence, we shouldn't kill them, and on the other hand we shouldn't just sit around doing nothing. The point of any punishment, I think, should be primarily to try to turn the criminal mind into a non-criminal mind, by psychological rehabilitation if necessary.

Locking up violent criminals into cages doesn't really help society, especially when they're often let back into society after several years of incarceration that just made them angrier and more psychologically imbalanced as their emotional needs were completely ignored in the cold heartless environment of the prison. That can only make things much, much worse.

The best way to prevent murder, I think, is firstly by combatting social conditions like poverty and homelessness which drive people to violence, secondly by providing free psychological care to mentally unbalanced individuals, and thirdly by distributing ganja as much as possible so people chill the fuck out.