Unfortunately they have the right to say what they will. They second they stop being just words and turn in to physical actions is when there is a problem.
"What’s wrong with hosting a manifesto? It should be blindingly obvious: when they spread extremism and terrorism like a virus, there’s plenty wrong."
Mein Kapmf is still published and is available online as well. I'm sure most people would agree that the view expressed by it's author were pretty extreme. No you keep that stuff up so you can show the world what a waste of space the author was and learn from it.
"This stuff happens in the US because the easy access to guns allows murderers to act like this. It does not matter whether 8chan falls now or whether it survives or whether the US limits their extreme stance on free speech as long the US society continues to accept that those massacres happen in favor of having guns available to everyone."
We will never surrender our weapons. Sure some may, but a good majority of us will not. We had a European government here once, but it became oppressive, so we booted them out. We did so because we had weapons to do so. We have the 2nd Amendment because we want the government to fear the population, not the other way around. Unless you want the U.S. to look like Alepo, it's just best to not go there.
Gun's are not the problem either. The problem is people, and total lack of respect. Everyone wants to force everyone in the country to love everyone, which is nice, sure. But how can you accomplish that when you can't even respect your neighbor.