Killer Cop

"The cop's the killer!"

—Brian (and probably everyone else on this page) in Psycho Cop Returns

This is pretty much Exactly What It Says on the Tin: A member of law enforcement who also engages in murder. This person's motivations may range anywhere from monetary gain to exacting a form of street justice. Either way, expect this person to be much more dangerous than your average killer.

Note that despite the name, the Killer Cop isn't always an actual police officer. He (or she) may also be a civilian employee, such as a forensics expert or something. Also, if the Killer Cop is an officer, their murders are obviously committed for a reason other than to maintain a cover identity. (Well, at least one hopes an undercover officer doesn't go to those lengths.)

If the Killer Cop is a Serial Killer, the observant viewer may recognize the following signs:

The crime scenes are jarringly devoid of any incriminating evidence. Either the killer is a forensics expert and cleaned up so thoroughly that they got rid of the evidence or they are a savvy police officer who has investigated many crime scenes and knows how to avoid leaving evidence in the first place. Note: This is not always a given, as a Serial Killer who isn't a member of law enforcement can avoid leaving evidence as well.
If the killer is suspected to be copying the methods of another killer, the copy will be exactly like the original, even down the details that only a member of law enforcement would know.
If the killer is attempting to frame someone else for their crimes, the frame-up will be suspiciously thorough. The amount of evidence against the person will likely lead the viewer to two conclusions:
1. That a calculating TV Serial Killer wouldn't be stupid enough to leave that much evidence against themselves, meaning the suspect isn't the killer and
2. The real killer has such intimate knowledge of evidence and police work that they must be a member of law enforcement. Like the first sign, this isn't a given, as it is possible to research forensics.

In Crime Dramas, may be used as a Twist Ending.

Note: A character can qualify for this even if they don't use their position or expertise to help with their murders. (However, if they appear in a murder mystery, they probably will.) See also Detective Mole.

Examples of Killer Cop include:

Comic Books

Americop from Captain America (comics).

Film

The eponymous characters of the Maniac Cop and Psycho Cop films.
Al Pacino in Righteous Kill.
Jim Belushi's character in the movie Gang Related. He uses guns already tagged as evidence in other crimes to rob and kill drug dealers. Ironically, the one character he doesn't kill is his partner, played by Tupac Shakur.
The vigilante cops from the Dirty Harry sequel Magnum Force.
Strange Days
The killer in Bloody Murder 2: Closing Camp turns out to be the sheriff, who is really the father of the thought legendary camp boogeyman, Trevor Moorehouse.
In Holla If You Hear Me, it's revealed there's three killers, two brothers and one sister. One of the brothers is a detective.
The Prowler turns out to be Sheriff George Fraser.
The Australian thriller Dangerous Game. Notably, the officer (while obviously unhinged) only killed by accident, and feels extremely conflicted about about trying to murder the witnesses.

Literature

Robert Crais seems to like these: In L.A. Requiem, the killer who was framing Joe Pike turned out to be the guy who pushed the mail cart at police headquarters. In Demolition Angel, the person who murdered Charlie Riggio was his own partner on the Bomb Squad, Buck Daggett. In The Two Minute Rule, the man who murdered the four officers was William Cecil of the FBI Bank Squad.
The protagonist from Jim Thompson's novel The Killer Inside Me.
The FBI Agent Denton and his cohorts in Fool Moon.
Frank Ennunzio, FBI Forensic Linguist in Lisa Gardner's novel, The Killing Hour.
In the Tom Clancy novel Without Remorse, a Corrupt Cop murders a drug dealer that his (criminal) boss suspected of being a security risk. He even manages to do it in a way that causes multiple witnesses - police officers themselves - to honestly believe that the victim was shot in self-defense when he attempted to resist arrest.

Live Action TV

Law and Order and its various spin-offs have featured these on more than one occasion.
Rizzoli and Isles had a detective who killed a bunch of women with the same names as the original Boston Strangler victims. His purpose in so doing was to frame the person he believed was really the Boston Strangler. He had worked the original murder case and didn't believe that the person who confessed was really the killer.
Of course, Dexter is one of these.
- He also went after one in season four, a cop who'd killed her own husband and daughter and made it look like a robbery gone foul.
CSI Paul Millander, the bathtub killer, is revealed to have a double life as a judge.
- Ray Langston, at the end of season 11. Not a cop, but still a CSI.
CSI: NY had one of those, a cop that went dirty and murdered others to cover his tracks.
In Criminal Minds, the first episode that features The Reaper leads you to think that this might be the case. The Reaper was a serial killer who had stopped killing for ten years, then started killing again. In the original run of murders, he had killed a bunch of people, but left one victim alive. Ten years later, that victim was hiding from The Reaper by living at a number of different addresses and switching between them. At one of The Reaper's crime scenes, he left the victim's address numbers written on windows in blood, leading you to think that the lead detective from the local police department might be The Reaper.(Since he would've been one of the few people who knew the addresses, as they had been told to him by the victim.) However, this turns out not to be the case. In reality, the "victim" survived because he was The Reaper.
- Another episode features a cop that stages shootings so he can be the first to "save" the victim. Occasionally those victims die, but he is honored as a hero cop. He tries to kill Peneloppe when he thinks she is on to him.
- In "Broken Mirror" it turns out one of the local FBI agents helping with the investigation is the perp, while "A Rite of Passage" has a Sheriff's Deputy turn out to be "Santa Muerte".
On Bones serial kidnapper (and also killer) The Gravedigger turned out to be United States Attorney and former FBI agent Heather Taffet, and chose self-representation during the trial.
- Also Agent Kenton in "Two Bodies In The Lab".
On Frasier in That One Case that Martin has been working on for years, it turns out that the detective did it.
One of the 1970s-era Columbo mysteries involved the Police Commissioner himself trying to get away with committing murder.
A recurring villain on Cold Case was a serial killer who worked as a civilian in the police department's records division, and used what he knew about the detectives to successfully deflect their interrogation techniques.
In an episode of Beyond Belief: Fact or Fiction?, the protagonist of one story is a cop who discovers that he killed a young woman while sleepwalking. He is completely unaware of this until he compares the bullet found at the scene to one in his gun.
An episode of Angel featured a police captain suppressing crime in his district by raising recently deceased police officers as zombies and having them indiscriminately and violently harass people on the streets after dark. They end up committing several murders themselves.
Detective Rick Messer aka The Bearded Man in Damages, who acts as a hit man for a Corrupt Corporate Executive. Plays with this as it isn't revealed that he's a cop until late on.

Video Games

Detective Adachi from Persona 4 qualifies as one of these. Bonus points for the supernatural powers that helped him avoid leaving evidence.
In the Adventure Game Laura Bow 2: The Dagger of Amon Ra, the killer turns out to be Ryan O'Reily, the detective working on the case.
Kurtis Stryker of the Mortal Kombat series.
Case 1-5 of Ace Attorney has former detective and curent high prosecutor Lana Skye accused of murder. The real killer is not just a killer cop, but a killer police chief.
- Not to mention killer prosecutors Manfred von Karma and Godot and Jacques Portsman and killer defense attorneys Calisto Yew and Kristoph Gavin.

Webcomics

Axe Cop. He'll chop your head off!

Real Life

There was a case once where the murderer turned out to be one of the cops working on the case. This was discovered after a visual reconstruction of the offender was created.
There has been speculation that the Long Island serial killer is someone working for law enforcement.
Serial killers Gerard Schaefer and Norbert Poehlke.