Unfortunately, people with the desire to "Kick ass, sleep 'till noon" (to use a favorite quote from Travolta in The General's Daughter) tend to be drawn to occupations that put them in positions of power. Psych tests for the police - particularly the state trooper tests in many states, used to be a lot more strict. That is, until they were unable to get recruit quotas filled.
A good part of it is training, a good part of it is the person. Regardless, the important thing to remember as a civilian is to NOT put up any protest at all. If you feel you're being mistreated, suck it up and complain later. Resisting arrest will not comfort your family when you are dead or in the hospital, even if you were in the right.
The vast majority of police officers are good people, putting their lives on the line every day for little recognition, a decent pension plan and a chance to take their car home if they work overtime. But the news will naturally seek out those bad apples that, when things go bad, result in people dying or getting hurt.