The problem with those ostensibly rare "bad cops" is that the supposed "good cops" tend to look the other way at best, and tacitly support them at worst. Cops who actually try to call their colleagues out tend to be heckled or worse. Here's a documented case of an entire police department ganging up against one whistleblower to the point of involuntarily committing him to a mental institution, with full knowledge, approval, and cooperation of higher-ups all the way to police commissioner.

https://en.wikipedia.org/wiki/Adrian_Schoolcraft