2) A cop who arrests another cop in violation of the unspoken code called the Thin Blue Line, which encourages police to turn a blind eye towards internal police corruption.
3) The best cop.
2) Frank Serpico was a killercop. The New York City detective who, after turning in a large number of "bad" cops and later being shot in the face, often went undercover to expose corruption in the city. The scruffy-looking, long-haired officer used unorthodox means to get results, and reported to Lieutenant Sullivan.
3) Federal Bureau of Investigation director Louis Freeh was often referred to as a top cop, or a killercop.