Under what circumstances can police brutality be justified?

I think the answer to this question may lie in the 14th amendment to the Constitution but as we all surely know, many court cases have interpreted the law differently over the years.

I believe police brutality, either as excessive force or lethal force, may be justified for people who refuse or resist arrest with a violent or likely violent reaction, potentially causing physical harm to an arresting officer(s) or citizens who may be nearby gawking at the developing incident. I also believe that the use of racial slurs can be considered police brutality because it can harm a person mentally.