A good cop is a dead cop.
I wouldn't go that far, though.
Three of my kin are good cops. My great grandfather and his brother were good cops (Well, Texas Rangers, which was the early form of the State Police, there).
They have an impossible job to do and they are hindered by human qualities. The most Perfect cop must constantly make trade-offs to balance the law, their suspicions, the well established (although, racist in some cases) profiles of those they question. There is no perfect law. There is no perfect cop. There is no perfect judgment. There is no justice.