Dutch resistance members celebrate at the moment they heard of Adolf Hitler’s death

That's precisely the point; sociological studies indicate that many such acts happen often as a result of a community. Muslim suicide bombers would be less frequent if they weren't part of a big community that glorifies the act, for example. Ditto for alt-right shooters at abortion clinics. Those things usually happen because a community talks itself up so much that at least one of the members decides to go for it.

Ever seen a group of teenage boys talk themselves into doing stupid shit? The average IQ of the group takes a nose dive when they're all together and there's no voice of reason.

I think what you see here is more a side-effect of Reddit policy meant to ensure such groups don't find purchase in their own special subreddits.

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