I'm pretty sure more Twitter and Facebook users have become mass killers than 8ch users. The first comment to the guy on 8ch who posted the manifesto was "hello FBI". Not a 8ch user but they don't appear to support shootings. I'm more familiar with 4chan shenanigans and they definitely don't support it, with any threat of real-life violence being met with something along the lines of "[alphabet agency] fuck off". Not…
The origin story of 8chan from what I understand is that it is all of the people who had too extreme of views for 4chan... which created an echo chamber to further radicalize and ferment.