I understand what you are saying but it cannot fall on existing communities to accept people who doesn't align with the communities goal.
And to your real question: its nothing different than saying movies and video games inspires a CRAZY PERSON to do harm. I can read 'Mein Kampf' without building gas chambers, and I'm sure you can too.
The real question is whether or not the shooter who frequents 8chan/2chan/voat/etc would have committed the shooting if they didn’t frequent extremist sites?