Cops who kill?

I don't understand why this keeps happening all the time. I mean no one wants to die, but cops need to relax..

They are not good cops when they kill people because of their race. They ought to be in prison as well if they have done wrong.

This is quite natural in U.S. I posted an article also on this. Early in the morning two Cops suddenly stopped their jeep and shot a person who was on the morning walk. They demanded him to get into the jeep. When he questioned he was shot. He was a black. Really the department turned out to be quite disgusting with such incidents to talk about.