I’ve known only one person who had been a policeman. Once, I asked him why he quit after about 10 years on the force. I think his reason is related to the Ferguson controversy.
He told me how for his first 8 or 9 years, he worked the night shifts in the town's worst precincts. His early training and experience made it clear that, late at night, there are much fewer people out on the streets in general, and most of them are up to no good. After enough years on the night shift, his attitude became "standard blue" as he called it, that everyone he ran into, day or night, was probably a "perp".
After nearly a decade of night shift, he became eligible for working days. He eventually learned that policemen in his town were deliberately kept working nights until they developed that "standard blue" mentality. As their careers developed, all the police had acquired the notion that non-police were to be considered "perps". This attitude was considered essential so other policemen could count on you in case of trouble, and it was also essential if you wanted to be promoted. After about 10 years, this friend began to doubt this way of thinking, and resigned.
I've never had a bad experience with police, but I've always remembered that story whenever I see a police car. I also think this "us against them" mentality may help explain some of what we're seeing in Ferguson. It doesn't explain everything, but it is part of the story.