I can't say about communists, as there are so few around these days.
I dated one. Her description was basically the "People's Front of Judea" scene in Life of Brian — they're all in their own tiny rabbit holes, completely convinced all the other tiny groups are pawns, puppets, or stooges of capitalism.
There are many reasons to read something other than "I am reading this because I love and agree with every aspect of it". In fact, it is probably common knowledge that only ever reading stuff you wholeheartedly agree with is quite a bad idea. Echo chambers, social media "bubbles" and all that. Also, it can be good to build a tougher skin, can't it?
Are you seriously claiming that people should visit /b/ to avoid echo chambers? Have you been to /b/? I'd argue that it's probably more productive to change your perspective by hitting yourself in the head with a hammer. You're not being freed from your echo chamber by listening to people who support genocide.
My comment is meant more as a defense of 4chan as a whole. I have seen valuable information originate from their technology board, for example. I just don't want to see prejudice against any and all 4chan users. Blanket application of "guilt by association", which seems to be advocated in neighboring threads, seems horrible to me.
I just hope people will be willing to engage with any other person up to the point that they actively prove to you in person that they are a jackass. Trying to look for signs that the other person is secretly a jackass is not a great approach.
So you hit it right there.
Neither fascism nor communism necessarily include mass murder.
Naziism does.