I find a lot of it is selection bias -- because the police generally interact with criminals, they come to view the whole population as criminal. Do it long enough, you start to get jaded and ask yourself "why do I follow the law when all these people break it with few consequences?"

Also see the effects of the "thin blue line" eroding institutional morality... it's easy to get to a place where we are -- even white people in America no longer trust the police.