It is impossible to be a "good" police officer because the system of rules and regulations they enforce is, first of all, fundamentally unjust. Any laws or regulations which may be considered "just" are not enforced equally amongst different classes, races, sexual orientations, or genders. Anyone who chooses to obediently enforce such a fundamentally unjust system is not "good". Anyone who is "good" would never choose to enforce or perpetuate such a system and wouldn't be an officer for long. Anything that could be considered morally good done by a police officer will quickly be offset by their actions taken to perpetuate an unjust system.
Further, the fundamental responsibility of the police is to defend corporate capital and fill for-profit jails that are still essentially debtors prisons that put people, especially minorities, into a cycle of eternal debt and oppression. They are the lackeys of a state owned in full by capitalists, and in most cases they are (by design) too stupid to see it. If they are not too stupid to see it, then they enjoy inflicting violence with the authority vested in them by public trust. If they are neither too stupid nor especially violent, then they are simply ignorant.
People exist on a spectrum. Therefore police do as well. But the point is that all cops agree to enforce laws which are not equitable. Very few of those laws have anything to do with safety and are in fact designed to enhance the comfort of the bourgeoisie.
