Edit for clarity: I'm not asking why the Tankie/Anarchist grudge exist. I'm curious about what information sources - mentors, friends, books, TV, cultural osmosis, conveys that information to people. Where do individuals encounter this information and how does it become important to them. It's an anthropology question about a contemporary culture rather than a question about the history of leftism.
I've been thinking about this a bit lately. Newly minted Anarchists have to learn to hate Lenin and Stalin and whoever else they have a grudge against. They have to encounter some materials or teacher who teaches them "Yeah these guys, you have to hate these guys and it has to be super-personal like they kicked your dog. You have to be extremely angry about it and treat anyone who doesn't disavow them as though they're literally going to kill you."
Like there's some process of enculturation there, of being brought in to the culture of anarchism, and there's a process where anarchists learn this thing that all (most?) anarchists know and agree on.
Idk, just anthropology brain anthropologying. Cause like if someone or something didn't teach you this why would you care so much?
Being frankly honest, this is just the history of reprisal and counter reprisal starting from the Russian revolution. If you read my post here and look into the history of the Russian civil war, it's very plainly spelled out.
The Bolsheviks criminalized this as wrecking which allowed them to propagandize future Marxist-Leninists against understanding a more neutral version of the history of conflict between the two groups. Many anarchists feel exactly what you describe here about the Bolsheviks actions towards Maknovischina during the Revolution -- the only real difference is that the Bolsheviks were successful at snuffing them out.