The "bad" boards frequently leak, and you do see highly political or racist posts from time-to-time on the regular boards. A sign that the community is functioning properly is regulars on /out/, /lit/, etc. calling out (read mocking) ethno-nationalist threads, or race-baiting for what they are.
The answer is structuring society, socialization, and culture in a way that doesn't disenfranchise people or leave them feeling helpless enough to turn to extremism, hate, and violence.
But that requires empathy and effort, things often in short supply.
People only go down the dark paths that lead to 8chan et al when the avenues to belonging they were presented with by their parents and by default failed them.
