Radicalizing terrorists to such an extent that they actually go through with it -- multiple times -- is extreme enough to make me wonder.
The El Paso shooter's manifesto was posted on 8chan for a reason. The manifesto was written specifically for other potential shooters. It contains tips and advice. It explains the shooter's choice of gear and that he needed to use heat-resistant gloves because one of the guns he used was prone to overheating if fired rapidly. It encourages potential shooters to avoid heavily guarded areas and to avoid engaging with security personnel regardless of how confident they are.
Throughout 8chan, I saw many posts subscribing to the general theory that racial violence has been accepted throughout most of human history and the only people who disapprove constitute a tiny little blip on the grand tapestry. The whole point of this ideology was to encourage people to commit acts of violence by indoctrinating them into the belief system that all their ancestors would not only have approved but would have done it themselves.
Mass shooters are a problem that must be dealt with. Pretending that everything to the right of mainstream Democrats is a pipeline to mass shooters that must be silenced is the type of thing that radicalizes people. It's unduly oppressive and it dehumanizes people you may not happen to agree with.
However, one of the largest social media sites out there (reddit) does NOT have to allow them to congregate together. Reddit should have banned them years ago. They are actively responsible for providing a place for hate to thrive.
You can be 100% pro free-speech and still agree that no one (except the government is some form, meaning sure they can hold meetings in public places, etc) HAS to give them a platform. Anything private can and should say NO.
My knowledge of human history mostly validates this theory.
It should be obvious to state that if you get rid of 8chan, those people aren't suddenly deradicalized and they're still in the country holding the same beliefs. Those people who are today at risk of continuing in this shooter's footsteps have already read the manifesto. People like the shooter often do these things because they want to be heard and they want to contribute to the course of history. Taking away what little voice they have in their own spaces makes them feel less heard. Destroying the little community that they have removes their stake in the world. Suppressing their only place to express their grievances causes them to lose hope. This is the cocktail for more violence, not less.
The *chans are the furthest from centers of indoctrination because the moderation is the weakest of all social media platforms. All ideas are present and little to nothing is suppressed. Dissent is commonplace and general consensus has no power. Sure, it means ugly ideas get spread, but it means that people actually have to refine their moral argumentation. No longer can you assume that the other person has the common ground of "racism is wrong", you have to actually dive into why racism is wrong. You can act outraged all you like, but it won't convince people to change their mind. Changing minds is going to be the only effective course of action to avoid tragedies like these.
More importantly, you should ask yourself how they found 8chan. They were using other means of communications that didn't disappear. The invisible force that gathered them together on 8chan is still there and can definitely do that again.
That being said, I grew up reading TOTSE, which might by this conversation's context in today's world, also sound like a source of the same.
See these examples:
https://nypost.com/2000/08/16/howard-stern-caller-busted-in-... (2000s; a caller threatens to kill Senator Lieberman on air)
https://www.reddit.com/r/todayilearned/comments/665af0/til_t... (Mid 90s; Stern plays gunshots during a Selena song)
https://variety.com/2014/voices/columns/howard-sterns-white-... (Stern books the Overland Park Jewish Community Center shooter)
https://www.howardstern.com/show/2016/10/24/ronnie-limo-driv... (2016 Twitter spat with implied threats of violence with Stern-affiliated staff)