Well, mass-shootings wouldn't happen if there weren't people who believed it. It turns out that anonymous 'edgy' internet forums are an ideal breeding ground for that sort of ideology.
The absurdly offensive dialog that goes on there is often just an act people put on to fit in with the crowd in a consequence-free environment--most of them are probably just harmless lonely teenagers with nothing better to do. But it would be hard to tell at a glance which posts are serious and which aren't.
Harmless lonely teens who may be play-acting on 8chan are encouraged to become true believers by the rest who are sincere. The shooter involved in this case stated that he’d only been on 8chan for a year and a half and had learned much in that time. At some point the “fun” of it crosses a line for minds that have a strong enough foundation to leave it behind, those that stay become invested in the ethos. They embrace the allure of being the righteous underdog in a battle for the “future of white America”. Empathy and compassion are the enemy.
History is overflowing with examples of people, groups, and nation states preaching, and practicing that crap.
As a kid, I heard similar stuff from my classmates and their parents and grandparents, and I was in the suburban Midwest, not a place strongly associated with anti-semitism at the time.
Today's purveyors have just been offered more powerful media to amplify and propagate it.
It's worth noting that this board on 8chan makes it an open policy to permanently ban people who post dissenting opinions with the reason that they are "shills" and coordinating a misinformation/propaganda attack.
(Warning: 8chan...)