ACAB doesn't mean every single cop on an individual level is a morally bankrupt shithead. (a lot of them are, but that's beside the point).
even if you are the kindest samaritan cop in the world who never hurt a fly, being a cop means that you are a part of the system that is designed for violence, for oppression and inequality and has a long bigoted history that spans to this day. you choose to associate with it, even if you don't realize the implications.