I figure there are roughly three different reasons anyone joins a police force:
1) It just runs in the family. Same is true of firefighters, it often just becomes a family legacy, and feels like a duty to join. 2) Probably most rarely, you get a truly altruistic guy who wants to try to better society and get thugs off the street. 3) You get a loser (very often someone who was always in trouble with the law themselves when they were younger) who goes on a power trip and orgasms over authority they're given.. essentially a bully.
I've met all three types. Number 3s are scary, but not representative of all or even most cops. If they were, we'd hear of a lot more incidences than we do, since there are roughly 1.1 million police officers in the US.