What would be done with him in an ideal society?

Probably not so much? In an ideal society he wouldnt be able to order people around to do bad things for him, he wouldnt have money or power to pay people to do bad things for him, he wouldnt find platforms that would help him create a following, he wouldnt find many people actually joining his cause etc. So in the best case he would just give up or constantly fail and be a sad old man that soon dies because of age.

But if he actually poses a problem with his anti social behaviour because change isnt something that he wants or can do and his actions seem to work to a degree, communities could try things such as:

• socially isolate him in ways that make it hard to built any social power
• monitoring his actions to intervene before he does violence
• remove access to things he could use to do violence
• exile him to a place where other bad actors live, like a a small town that can mostly sustain itself but gets some outside help to not be a death sentence
• and lastly, trying to dominate people that maybe liberated themselves, with many maybe having lost friend, family, community members during their liberation, is a recipe to get oneself killed out of justified rage or through simple self defense.

A more sophisticated answer is provided in the anarchist FAQ