America has a problem with police killing people. They kill a disproportionate number of people compared to every single first world country. US Police self reported to the FBI that they killed 400 people in 2012, this is only those that were reported to the FBI the number is surely higher. In the same time police in Britain killed 0, German Police killed 6 in 2011 and Australia killed a whooping 105 over a period of 22 years. These are all large nations with diverse population and huge numbers of immigrants. Why do you think police in the US are worse than other developed countries?
Partly it's because this is America and we're "at war" with everything. We have a war on drugs, a war on crime, a war on organized crime, in some places even a war on prostitution. It's a useful rhetorical device until you realize that, crap, people DIE a lot in wars, so declaring war against the entire portion of your population that uses recreational drugs is probably a really shitty thing to do.
Mostly, though, it's because police departments have become repositories of ex-soldiers and/or wannabes who couldn't handle a military career and decided to do the next best thing. An alarming number of these people are, in fact, cowards and instinctively hide behind the threat of violence and/or firearms when they feel their authority is threatened.
Violence is the last refuge of the incompetent. American policing jumped that shark in the 60s.