Being a police officer requires a certain amount of violence and aggression, and should not be a barrier to promotion unless the officer consistently demonstrates an "excessive use of force" when dealing with people. If a violent, aggressive police officer has a troubling record and has been the subject of multiple investigations, I would not recommend a promotion without first requiring the officer in question to undergo extensive therapy.
It's a tough call. Police offers have rules they must follow. Whenever an armed person waives a gun at police officers, and are asked to "drop the gun" but don't, these aggressive individuals will be shot at. Often, these individuals die. Recently, there were two such cases, Sterling and Castile, although there is speculation that Sterling did not have a gun. Cameras capture everything, so if there is wrongdoing, these violent officers will be punished. This is little consolation to the families who lose a family member (for nothing it seems). Police officers should be promoted when they stop crime for not using a gun. The taser is an option. Talking a would-be criminal out of a crime is another useful and admirable approach. The goal is to fight crime without killing people. It is harder not to use a gun than to use a gun. There is protocol. Violent people are warned they will be shot at -- stupid people disobey and get killed. What is alarming is if the person obeys and gets shot at anyway. These cases must be looked at individually. The FBI usually gets involved.
Police violence is a very real problem in today's society, and officers who lack the good judgement to remain nonviolent except when absolutely necessary during the course of their duties should never be given promotions. This sort of thing just results in a culture of violence becoming even more ingrained in today's law enforcement agencies.
If a police officer is known to be/considered to be violent, they should no longer be a police officer. Rewarding an officer for violence just promotes the problem. Any man or woman who has shown violent tendencies towards others is not fit to be a police officer, no matter what!