Good cops can only exist until they come in contact with a bastard cop.

When a police officer beats someone to death or shoots an unarmed motorist or steals drugs from the evidence room, there is often at least one witness: Another cop

At that point the good cop either becomes complisit in the cover up and is no longer a good cop, or they report it in which case they will be excluded, fired, harassed, sent to dangerous situations without backup, or die under unusual circumstances.