The wrongful police killings that happen in America are mistakes: the policemen is careless, shoots a bystander, raids the wrong house, etc. There are also the wrongful killings that are not the fault of the policeman: the victim points a toy gun at him, or moves to pull something out of his jacket while being arrested. There, the solution is not to put the policeman in prison, but to make him personally liable for damages and to fire him. Trying to go the criminal route ends up with zero penalty instead, because it seems too extreme and because it is hard to convict for a mistake when the standard is beyond a reasonable doubt, particularly if the defendant is trying to to his job as opposed to recklessly enjoying himself. Moreover, the most important thing in such cases is to incapacitate the offender. That means firing him from his job as a policeman, so he won't get a chance to make the same mistake again.
It would be interesting to look at a random sample of killings by police and see what happens. We already know that there are very few convictions, and that doesn't tell us much. What is more interesting would be to find out how often the police department or policeman (or does he have immunity?) pays damages, either with or without a formal lawsuit, and what happens to the career of the policeman when he is at fault.