Earlier quoted context omitted.
The goal is not to convince conspiracy theorists that they are wrong, the goal is to prevent them from hurting people or society and, to that end, keep the dangerous ones from organizing anonymously online in large numbers (I honestly don't know if that's possible).
If I'm not mistaken the NZ killer was on 8chan? It's the same argument with serial killer movies (at the risk of showing my age there was a strong controversy with "Natural Born Killers" at the time): you don't kill people because you chatted on the internet or watched a movie. You kill people because you had it in you. The things you were exposed before might color your act but that's it.
There is this pervasive idea lately a crazy youtube video or text can't actually be harmful, which just isn't true. Ideas can inspire wars, murder, revolution, they can warp minds. People previously productive can end up obsessed with crazy junk. It is important to protect each other and ourselves from bad ideas. The question is how, of course. Some people violently oppose any form of curation, whether it be censorship by government, or choices made by companies like facebook. Others insist the solution to bad speech is good speech, but I've been seeing that fail on the internet for 30 years. Perhaps higher quality education can help, perhaps improving economic inequity can help. Other ideas are welcome!