1) A Citizen who attempts to or arrests a cop for breaking the law, without getting killed.
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.
1) killercop was the name of an Internet website owned and published in 1997 by Steven Sutcliffe. The name killercop was coined by Sutcliffe in 1997. killercop.com contained Sutcliffe's story about police corruption and numerous other claims of official corruption, misconduct or otherwise illegal activity. The site became notorious of its citation of an early US Supreme Court Case, US vs. John Bad Elk, which stated the acceptability of killing an officer during an attempted arrest when the arresting officer had no legal authority to make the arrest. In conjunction with this legal citation, the website offered a parody reward for individuals who justifiably killed a police officer in the course of an attempted 'illegal' arrest. The site then offered additional legal reference materials to help readers determine what constitutes an 'illegal' arrest.
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.