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Bellingcat on the 8chan inspired "for the LULZ" elements of the fucker's manifesto and music choices in the stream etc.
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FFS, only yesterday Behind the Bastards dropped an episode examining some of the same stuff. It goes into some detail on the legacy of the book 'The Turner Diaries' which according to Bellingcat has inspired today's terror attack.
Robert Evans describes how this legacy has inspired numerous far right terror attacks and only hours after the episode is uploaded that exact thing happens.
People who were kicked out of 4chan for being too extreme. Lots of child pornography, white supremacist stuff etc. The point being, 4chan is already pretty fucking gross and these people were deemed too much for them.
(and @dubtap in case you were interested)