I have to say right off the bat, I'm not in favor of governments or big corporations being gatekeepers of what is allowed to be on the internet (moral hazard).
At the same time, views not in accordance with actual history (or indeed with reality) propagate at an amazing pace and certain subgroups readily embrace them as reality.
In case of shooter (reading his manifesto), he has a juvenile and one dimensional view of "white history" and "white culture" which fails to understand or account for movements of ideas and culture, and fallaciously lumps everyone of a certain skin tone into one cultural group. I have no doubt this view, fanciful as it is, was echoed by his fellows until it became absolute truth in his mind.
It is a problem, and not just for white supremacist types and radical Islamists either. Not sure the best solution or if there even is one.
It’s a very hard problem. The printed press indirectly generated the Protestant schism, which resulted in centuries of wars and murderous events. The internet is having an equivalent effect today.