You're a bit off in the ACAB definition. It's not that the state makes them do bad things (they do, but that's irrelevant). It's that all cops protect bad cops, making themselves bad cops as well. If a department has 30 cops, 3 are "bad" and 27 are "good," then the bad ones should be forced out. That doesn't actually happen though, so you have 30 bad cops.

This system is also heavily self reinforcing. If you add a 31st good cop that tries to do something about bad cops, they are either forced out or intimidated into compliance. That still leaves you with all bad cops.