The decision to kill would an act of defense. Organized sex trafficking preys on members of a community. If you see it happen, stop it. If that means killing the perpetrator, you’d need to justify it to your community, but you don’t have to kill them to stop it. If you’re going to go after the organization, then the course of action would be decided by those impacted by the organization, either by vote or by consensus, whichever applies to your community. And that decision making process goes for essentially anything within a community. Again, check out democratic confederalism and anarcho-syndicalism for in depth explanations.
What you’re describing in the second bit is genocide, not fascism. Fascism wouldn’t manifest in a bottom-up society. It’s a hierarchical system. By the time a society devolves into fascism, it would have ceased being a libertarian society long before. As for how to stop genocide, just don’t do it? It’s a form of dominance, same as other things we’ve discussed. Stand with with oppressed, support their liberation and directly act to fight alongside them if you’re able to.
And I’m not that knowledgeable on cults, I’m sure someone out there has thought about it but that’s not me. Again, they exist now and we’re not doing anything about them. When the state steps in, they’ve gone disastrously. Jonestown comes to mind. Attempts to bring people out of them are by and large grassroots movements and non-profits. Both of those systems would thrive in a libertarian model and they would have a lot less red tape to contend with in order to liberate the cult members.
What checks to cults do we currently have? The LDS church, Jehovah’s witnesses, and scientologists are absolutely massive cults that (with the exception of the Jehovah’s witnesses) have infiltrated every level of government. Why hasn’t the state eliminated cults if they’re so capable?