A white friend of mine who I have known for a majority of my life was an Oakland Police Officer who was murdered while on duty by a black thug in 2009.
http://www.odmp.org/officer/19882-police-officer-john-raymond-hege
However that goes nowhere near touching what police have done over the years to the black community. Let's start with being profiled for walking in the wrong neighborhood while black.
As a white (or Asian) person you can walk in any neighborhood and a cop isn't going to stop you on the sidewalk and ask what you are doing here. They do that for black folks. People talk about the Jim Crow laws in the South but many northern and western cities basically had Jim Crow with cops harassing blacks who dared leave their neighborhood and go to the white part of town. What do you think that does psychologically to someone when that happens over and over?
Of course we have people being pulled over for simply driving while black and receiving abuse at the hands of the cops. We have too many to count instances of police brutality beating on black citizens for no reason. And then the number of cops that have attempted to frame and plant evidence on innocent blacks. There is a long history of this, not just isolated incidents.
I'm not coming from a position of all cops are bad or that cops don't have a difficult job or cops don't deserve any respect. But I am speaking the truth about the relationship between cops and the black community.