Exactly this. These sort of folk started congregating on 8chan when they got pushed out of 4chan. I certainly don't condone mass shootings, white supremacists, etc., but if you think denying 8chan hosting is going to solve this problem, you are sorely mistaken. All it does is force these people into more concentrated forums where there is less push back and opposing views to their radical ideas. Ultimately, it forces them into corners of the internet that are harder for outside groups to locate and monitor for potential threats.
I'll be quite honest, I don't know what a "good" solution here is, but I really don't think this is it.