If you are of a fundamentalist/authoritarian mindset, you see the world in such black and white terms that there are only two groups of people: people who believe 100% exactly what you believe, and Nazis/Libcucks/Kuffar/Satan/Reprobates/Kulaks/(((them)))/Degenerates/TERFs/whatever word your particular brand of crazy prefers for its enemies. It doesn't matter if they agree with you about 99.99% of everything, the second one of these people disagree with you about literally anything they are literally Antichrist TurboHitler and may as well be from the complete opposite end of the spectrum. I've seen people on SA say without any irony that any ideology other than their own is literally Fascism, and that the idea of political nuance is "toxic" and "a result of a lack of education".
Why yes this does seem to be the same behaviour that you get with Borderline Personality Disorder. I'm sure that's a coincidence.
And of course as these people have long since blocked everyone or retreated into hugboxes to get away from any genuinely differing opinions, they only ever interact with people who are of the same political or religious slant as them. So they pick their enemies from this group rather than opposing ideologies, which is why extremist and authoritarian groups, be they left, right or religious fundie, spend more time fighting amongst themselves than they do with the "enemy", and keep forming splinter groups based on the tiniest differences in dogma.
For example the number of Incel splinter sites is huge for the number of people who actually subscribe to that ideology, so it's not just the left that do this. Also witness the way the GG movement fractured at its more extreme corners. I'm not much of an authority on Stormfront, but I would be amazed if they don't have slapfights resulting in splinter groups as well.