I'm not overly knowledgeable on the topic, being neither American, nor having had any negative experience with police before, but when the Alton Sterling case made its rounds, I've read a, for me, very insightful comment about the whole police brutality/trigger-happiness issue. I'll try to paraphrase it as good as I can.

There are massive differences between areas/districts/states in the US in every aspect (that doesn't surprise anyone). One of these aspects is the training of the police force. It tends to be better (along with the general quality of education) at places where the rate/probability of violence is [i]already[/i] lower. Not to mention that the environment itself has an effect on its cops (dealing with dangerous cases day-to-day and fearing for your life is bound to make people anxious on some level). There are surely a variety of other factors too, but the main point is, simplified, that the ‘good’ cops are most often found in the already safer areas. And vica versa, unfortunately.

The commenter mentioned that they had an officer friend who was a great person and an excellent cop. He was a prime example of this: he had the decision to either give out tickets to soccer moms and hold educational speeches for the youth in a rich, suburban neighbourhood or deal with gangs, drug trade and all that jazz, risking your life at every step. The decision, unless you're either very bold or very dutiful, is a no-brainer.

Just my (and the commenter's) two cents.