I beg to differ. This is not about a police officer either acting or failing to act. This is about determining what is appropriate use of force and what is not.
Shooting a teenager into a car seat (and lying about it before getting caught) is very different from a situation where a police acts in self defence or chooses to use violence to stop other people from getting hurt. It's that simple.
A misconduct has to be punished and not rewarded. Always.
Any breach of police conduct should be investigated and judged on case-by-case basis. If an officer is able to justify his actions and they are reviewed to be according to rules, He should be allowed to continue working. If He has misused his authority He should be punished accordingly.
In a civilized, functioning country, this does not scare the rest of the police, but rather forces them to evaluate their practices and actions for the benefit of their work and the society. In the long term, this is much better for the society than just wiping misconduct under the rug.