Bad cops are the exception, not the rule. And they're not all that common. However, the nature of the job does tend to pull a disproportionate number of people who get off on the power, and the psych exams can't weed out all of them. And certain regions tend to pull and even higher proportion of those, as those with the hiring power tend to pick up more people like themselves. On top of that, problem cops are almost never fired, they're simply shuffled around between precincts, and often eventually end up concentrated in the "worst" precinct, usually the one in the "bad part of town". One of the places I grew up was notorious for having some of the worst city cops in the region; though the county sherrifs were far better.