This is an age old problem and I'm not aware of anyone who has given the problem of police violence more thought than Norm Stamper. Here is a Nation article that summarizes the problem very well from the perspective of a Police Chief: http://www.nationinstitute.org/blog/nationbooks/2492/still_breaking_rank...

The paramilitary bureaucracy and the culture it engenders—a black-and-white world in which police unions
serve above all to protect the brotherhood—is worse today than it was in the 1990s. Such agencies inevitably view protesters as the enemy. And young people, poor people and people of color will forever experience the institution as an abusive, militaristic force—not just during demonstrations but every day, in neighborhoods across the country.

Much of the problem is rooted in a rigid command-and-control hierarchy based on the military model. American police forces are beholden to archaic internal systems of authority whose rules emphasize bureaucratic regulations over conduct on the streets. An officer’s hair length, the shine on his shoes and the condition of his car are more important than whether he treats a burglary victim or a sex worker with dignity and respect. In the interest of “discipline,” too many police bosses treat their frontline officers as dependent children, which helps explain why many of them behave more like juvenile delinquents than mature, competent professionals. It also helps to explain why persistent, patterned misconduct, including racism, sexism, homophobia, brutality, perjury and corruption, do not go away, no matter how many blue-ribbon panels are commissioned or how much training is provided.

External political factors are also to blame, such as the continuing madness of the drug war.

That's the general idea. There are all kinds of youtubes and articles about and by Norm. Check it out!