I know this is hard for some people to accept, but the police in a free and democratic society are ultimately accountable to the citizenry yet some police behave as if they are only accountable to themselves. They shoot, club, pepper spray and choke people and yet only start to face justice for these actions as a result of a literal uprising.
Everyone accepts that the police have a hard job to do, yet the system has grown to shield bad officers from justice and reduce public accountability. This is a system driven by institutionalized racism.