Think about this: in most of the country, cops are the only people who get to walk around town wearing weaponry, so power-trippers are naturally going to want to be police. If the departments' don't screen those people out, then you've got someone that just can't wait to use their new billy club, stun gun, or pistol on someone.

Police training emphasizes officers that were killed in spur of the moment encounters or ambushed in planned executions, teaching recruits that they are alone and misunderstood, and that their only real friends are other cops.