For those who are unfamiliar what this is referring to, the full saying is "A few bad apples spoils the bunch." This refers to the fact that overly ripe apples give off a chemical that speeds the ripening process of other apples, so a few bad apples among many good ones can destroy them all.

The same thing can apply to people. The fact that there are bad police out there is only to be expected, just like you'd expect to find a few rotten apples in a large bunch of them. But just like the apples, you have to remove them, or the rot will spread. Bad police don't seem to get removed nearly often enough.