Have you ever seen what happens when you leave a bad apple in with a bunch of good apples? Very quickly, the entire bunch of apples is bad.

That's why we have the whole saying about "bad apples".

It utterly amazes me that people constantly use this "bad apples" expression without understanding this. Anyone who eats apples should have this basic understanding about bad apples.

Anyway, from what I can tell, whether cops are good or bad is basically department-wide: either a department has decent cops, or they're all horrible and complicit. It's exactly like the "bad apples" analogy, and it depends on the leadership of the department. Notice that it's usually the same departments that come up over and over in the news.