Way I figure it you get dicks from highschool that want a power trip, become the local badge'n'gun and sign up to a police department that simply does not have the resources to do proper training and psych evaluation.
Best way to solve this is to actually evaluate whether or not people are suitable for the criteria you desire in specific parts of the police force, and assign them accordingly. Trigger happy types should be on desk duties.
Even then, you'll have some that slip through the cracks. Your mildly above average sociopath that gets a kick from the power. This is why you need external oversight bodies, who can have complaints sent to them, and who are going to act on it.
What shits me is that a lot of cops do a damn fine job, and you never ever hear about it. Every day, there are plenty of cops who solve crimes, prevent crimes, help people out, etc. Hell, a friend of mine was given a lift once after he'd been mugged and didn't even have money for the train home.
However, none of that shit is terribly good column space in the morning paper. You're not going to buy a newspaper when the cover story is "Cop gives dude a lift home at 3am". You might pay attention if there's something about a major drug bust... But you're really going to fucking buy into it when the cops jack boot someone, regardless of whether they deserve it or not, because people like to read about things that don't normally happen.
Again "Cop gives homeless black guy a cup of coffee" isn't going to sell news, but I've seen it happen on more than one occassion.
So don't tell me that every cop is an arsehole when you've got departments that aren't even bothering to evaluate candidate suitability to an assigned role.
"Quis custodiet ipsos custodes?"
The Coast Guard... duh!
Simpson quotes aside, you can't lump all cops in together.