Cops are just regular people on a power trip. Soldiers work in far more dangerous situations and have better trigger discipline. In Afghanistan, plenty of times soldiers cannot engage an enemy of military age holding an assault rifle until that enemy aims at the American soldiers. Cops are unprofessional and have no good excuses for their behavior. If soldiers are tried according to the UCMJ for violating rules of engagement, cops should certainly be tried for murder.
If you think that crushing a man's neck was a mistake, how many minutes of crushing his neck would turn that mistake into intentional murder? If you think those three policemen guarding the killer were also bad apples, how many policemen would have to stand guard for it to be a systemic issue? If it's not a good time to protest police murders, when should we?