It depends on the totality of the circumstances. For example, whether or not the action even constitutes "brutality" is incumbent on the circumstances the officer faces, and the community or societal...
This is a very old problem; even in the Roman Empire, the question was famously asked, who guards the guardians? Since it is the police who enforce the laws, it is the police who have to arrest other...
Law Enforcement's answer to police brutality is to turn the other cheek; commonly known as the "Bleu Wall of Silence;" which, unfortunately happens ALL to often...