No brutality is legitimate. First of all, if police get this pass, then so do the street criminals they deal with. And then you just have a never-ending conflict since both sides get moral passes to put themselves above the greater good.

If you study game theory such as the prisoner's dilemma, you'll understand that these are perverse incentives where actors are certainly "rational" given their constraints, but the overall system is harmed. In a feedback loop such as society, this can have a runaway effect until eventual societal collapse.

> legitimising this is appropriate

For who? Who decides this?

> It is the responsibility of those in power to prevent society from degrading to a point where police are forced to be violent in order to keep their jobs.

Maybe you now understand why this is circular logic. If those in power are just doing what they "have to do" in order to keep their job and survive, and civilians do what they "have to do", and police do what they "have to do", the buck gets passed to no one. Every single culpable party gets to say it's not their fault or their responsibility to introduce structural change through personal sacrifice.