The online forum 8chan is better known than it was last week because the El Paso shooter, Patrick Crusius, uploaded his “manifesto” to the site before he murdered 22 people. 8chan has also been linked to the mass murder in Christchurch, New Zealand, and to a killing in April at a synagogue in California. A similar online forum, Gab.com, was allegedly used by the shooter who killed 11 people at a Pittsburgh synagogue in October. The Dayton shooter, Connor Betts, spent much of his time raving on Twitter.

8chan describes itself...