It's a really good movie that – despite its reputation among edgy tryhards – cleverly portrays the radicalization process which turns alienated, lonely men into terrorists and extremists of various sorts.
The narrator has a split personality in the movie, which is an extreme representation, but that kind of basic psychological "doubling" is a common theme among these guys. Tyler Durden is an idealized image of who the narrator wants to be, and you'll typically see mass shooters and terrorists engaged in the same kind of behavior; often through posing with guns in front of a camera (a common trope). It's like they're constructing this alter-ego which lives alongside the "normal" (albeit broken) ego, and the alter-ego eventually devours the original, shattered personality.
This is also why witnesses report mass shooters acting in a very calm and deliberate way. The new personality is dedicated to carrying out an apocalyptic "mission" so they feel really good about themselves and what they're doing. For the first time in their lives they feel like they have a purpose and they carry it out with ruthless, terrifying efficiency – although the actual results tend to be pathetic little murder sprees ending in their own death.