You can't tell the that it's not hypocritical to shut down 8chan because of this shooting, but last I checked ISIS still has a facebook page and twitter is filled with calls for political violence.
Besides, this is an entirely hollow gesture, and will achieve nothing of meaning. 4chan formed following a schism on 2chan, 8chan formed after the exodus from 4chan following Gamergate. The only question is which board the actually dangerous people will congreate on next, and how much of regular internet culture will be smeared in the process of playing this pointless game of whack-a-mole.
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I'm bored out of my skull, let's play a different game. Let's pay a visit down below and cast the world in flame.