Always in us, the land of freedom and equality or am I wrong? They are only poor bastards,? If the police wanted he could stop him without an useless kill (he could shoot in the leg for example).
Police are trained to shoot at the center of mass. They repeat it over and over until it's reflex. Once that cop made up his mind to shoot, the bullet was going to the center of mass.
So they are trained, but in this case there wasn't necessary to shot a man (in the back). He could shoot on the leg, and conclude the story with a simple arrest but all we know that the us police are thinking "I'm a hero".