What you described about someone taking on the police department kinda happened about 11 years ago:
https://en.m.wikipedia.org/wiki/Christopher_Dorner_shootings_and_manhunt
I remember the manhunt briefly making the news, but I don’t remember them covering his motivations, just his actions.
I would imagine it’s a multi-part solution:
You either are jumped in and prove yourself to be tough enough to roll with them, or you can’t tough it out but they’ve already beat you to a pulp once so ideally they’ve made their point and intimidated you out of speaking against them/talking to cops or trying out for rivals.
Full disclosure: I don’t have experience in this and am mostly speculating.