Thank you for your thoughtful, knowledgeable, and committed response, Alan. I'm really just speculating when I say there may be larger forces at work--much in the way that the Koch Brothers sponsored and encouraged the Tea Party movement for their own ideological and financial reasons. (I don't think the surviving Koch brother is sponsoring police brutality though.)

If you look at the incidence of lynchings in the south after the Civil War, it's pretty clear that the phenomenon is due to more than just the actions and attitudes of the lynchers themselves or the KKK. Perhaps there wasn't a formal organization or agency responsible, issuing formal directives, but some sort of collusion enabled lynch mobs to think that they should do what they did and that they could get away with it.

But as you say, there doesn't have to be a bigger conspiracy for police associations to think that they are entitled to do what they do and should be immune to prosecution.