He's not necessarily going "full-caveman" for the sake of it. He's spending the entire movie trying to restart from 0, and "finally hit rock bottom". Because to him that's the best way to build from. He spends most of the movie still not accepting his true-self, and can't come to terms with his feelings and emotions until the famous plot twist at the end. He (spoilers) needs to be somebody else because he can't possibly imagine himself with this woman he detests, and doesn't even realize he still sees beauty in her despite his disgust towards all her damage and flaws. 

The fighting ia from all the lost men in the movie trying to find purpose, because the urge to fight exists despite them no longer having something to fight for. Then they realize it makes them feel good and they hold no animosity to each other from the fights, they're getting out all that aggression. 

The vandalism-cult side plot isn't something the author is telling you to do, but it's just the culmination of the characters' desire to mock society. The Narrator tries to stop everything he's put in motion, and realizes after he becomes his true self that he's capable of being fulfilled and happy, even after all these economic hubs collapse in front of him, and he's lost his horde of treasure.