Serial Killer Killer

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Harry Morgan: "Son, there are people out there who do really bad things. Terrible people. And the police can't catch them all. Do you understand what I'm saying?"

Teenage Dexter: "You're saying... they deserve it."

—Dexter

A violent, psychotic killer with a Freudian Excuse gets sick pleasure out of the suffering of his victims. It sounds like he's your basic Serial Killer, right?

He would be, but instead of terrorizing the innocent, the Serial Killer Killer terrorizes the guilty. He spends his life tracking down serial killers so he can give them justice. In short, he's a vigilante, who thinks himself divine justice incarnate. Bonus Points if he kills them in the same way they'd kill their own victims.

Distinct from He Who Fights Monsters because He Who Fights Monsters is more about good characters turning evil in the process of hunting evil, whereas this is more about someone who is evil, or crazy, or both from the outset.

Arguably, this guy can be Chaotic Good and be an Anti-Hero, although he walks a VERY dangerous line to become the Knight Templar or, worse, a Complete Monster.

A Sub-Trope of The Hunter.

Compare and contrast with the similarly named Wife-Basher Basher, and note that these two tropes more often than not overlap each other.

As a Death Trope, all Spoilers will be unmarked ahead. Beware.

Examples of Serial Killer Killer include:

Anime and Manga

Almost all of the Kiras of Death Note fall under this trope, with the exception of the one profit-driven Corrupt Corporate Executive. Most notably, the main character Light is a mix of Knight Templar and A God Am I, believing he'll become the "god of the new world" by completely eradicating the world's criminals. True to the series' penchant of showing what happens to He Who Fights Monsters, a good number of the Kiras Jump Off The Slippery Slope into Knight Templar (such as Light himself) or just plain crazy (such as Mikami) territory.
Code Breaker. Some of the Code Breakers recognize that they are evil themselves, though.
Scar of Fullmetal Alchemist is pretty much this, except replace "serial killer" with "usually unrepentant war criminal", and does indeed kill them using the same power that they use to do their war crimes.
Diana, Selacao number 11 in Eden of the East, hunts rapists and dispatches them by castration via cigar cutter.

Comic Books

It could be argued that Rorschach from Watchmen fits in this trope, though he is more commonly placed as He Who Fights Monsters.
Suture from the comic Curse of Spawn.
Marv becomes this in the Sin City story, "The Hard Goodbye."
Cassie Hack in Hack Slash is a former Final Girl who becomes a slasher-hunter. In one of the later comics, she's even referred to by a talk radio host as the "SKK".
The Punisher is more of a Mass Mass-Murderer Murderer but he still fits.
Venom from Spider-Man (Depending on the Writer).
Night Raven, from Marvel UK (though his stories take place mostly in the US), has targeted serial murderers, including an unauthorized successor to his mantle, Howard Bates, who had admired Night Raven as a child.
Andrew Vachss wrote a Predator story where a Predator targeted serial murderers.
The Harris Comics series Twister featured someone who not only slew serial murderers, but did so by twisting their heads around.
Frank Miller's Batman. Regular Batman spends his life fearing that he may become this if he ever loses control.
The Scourge of the Underworld in the Marvel Universe is an entire organization of these - though only a scant handful of its victims are actual killers.
Sinéad Harkin and the servants of Mother Maiden in The Unforgiving (the base for Within Temptation's album of the same name) are very much this.

Film

Ben O'Ryan in the film Suspect Zero
The Crow anyone?

Literature

Agatha Christie's And Then There Were None: U.N. Owen. Except that most of his victims were ordinary murderers rather than serial killers.
Hannibal Lecter at times.
The main character of the novel Bad Monkeys claims to be a member of a secret organization devoted to killing people who are just plain evil and unredeemable. Of course, she could be lying. Or not.
Philo Vance in The Bishop Murder Case.
A.J. Holt's heroine Jay Fletcher in the two-novel series Watch Me and Catch Me.
Jeff Povey's POV Character in the novel The Serial Killers Club manages to be invited into the titular organization after he takes down a serial killer...and begins to kill off the membership, one by one. He says he's not a serial killer himself...but he kills again and again...
Both Shiki and Fujino in Karano Kyoukai. This is part of the reason why the part of the story where they fight is the only one where both agonists get out okay, more-or-less. (Well, that and the one where the antagonist has Mind Control to stop Shiki from attacking him...)
Some of vampires in Anne Rice's books tend to be this. Post-Akasha Lestat mostly kills and drinks mobsters, muggers and the like... but sometimes cannot control himself and kills someone he deems particularly impressive. Another vampire, Pandora, then we meet her, is hunting a drug dealer.
All of the victims in Regina's Song had criminal records to some degree, but the one the killer was explicitly looking for while she thinned out Seattle's rapist population was a serial killer. At her trial, one of the witnesses remarks that there was a perverse charm in one serial killer dying at the hands of another.
John Cleaver of the I Am Not a Serial Killer books is this, with the added twist that the killers he hunts are not human.
The Dexter novels go a bit further than the show--not only do we have Dexter himself, the two children he's raising are damaged in much the same way he is, and he's trying to teach them to be like him so as to prevent them from becoming even worse.
Edward Cullen from The Twilight Saga spent a few years doing this during his "rebellious" phase against Carlisle's vegetarian vampirism philosophy.
The murderer in the Kurt Wallander novel The Fifth Woman turns out to be a rare female example of this trope.

Live Action TV

Dexter, from the show of the same name, and the books Darkly Dreaming Dexter and its sequels. Dexter realizes that he's evil, but has programmed himself to live within a set of standards that are supposed to place him above common murderers. He sometimes allows himself to fantasize that he is a dark avenger of the innocent.
In The Inside, the episode "Prefiler"'s titular character, played by a pre-Lost Michael Emerson, profiled potential serial killers, tracked them down before they got a chance to kill, and killed them using their own intended methods.
Clifford Banks in Murder One is a subversion: although most of his victims were unconvicted serial killers, his first ever victim was his brother, whose murder he forgot and mentally pinned on a burglar. Ironically, this is what led him on the killer-killing path.
After Angel gets his soul back, Darla accuses him of being this when he tries to win her back. She says that while he has been killing, it's only been "murderers and rapists." Then she tried to make him kill a baby to prove himself.
In the very first episode of Tales from the Crypt "The Man Who Was Death" Niles Talbot is an executioner who was recently fired after the death penalty was abolished in his town. As a result, he goes on a killing spree, killing those who murdered people and escaped justice by various means of electrocution. However, eventually karma bites him in the ass when the tables are turned on him and he is executed using his preferred method just as it was reinstated.
Madame Vastra, a Silurian living in Victorian London in Doctor Who, eats Jack the Ripper, apparently with Inspector Abberline's blessing.
An episode of Law and Order Special Victims Unit has a woman deceive who can't handle that she killed a torturous murderer.

Tabletop Games

The Forgotten Realms of Dungeons and Dragons have this as the encouraged behavior of the priests of the little-known deity Hoar, Lawful Neutral god of retribution. Specialty priests of Hoar are called Doombringers, and are highly encouraged to kill or otherwise punish (as appropriate for the crime) criminals in a manner befitting the criminal's own misdeeds, especially if they can inflict an ironic punishment.
Similarly, Mage: The Ascension has the Tradition of the Euthanatos. The Euthanatos view existence as a continual cycle of death and rebirth -- "the Wheel" -- and those who unbalance the Wheel through atrocities must be dealt with so that things can be set right.

Video Games

Serial Killer X from the first Condemned game. Spying on FBI Agent Ethan Thomas, SKX figured out the identities of ten other Serial Killers and murdered them in the way they did their victims, which caused them to be confused for their own victims by the authorities and the investigation to go cold, thus keeping X's involvement unknown. In the sequel, his goals have shifted to dissecting people in an effort to give himself superpowers.
Haseo, the Player Killer Killer from .hack//Roots, .hack//GU. Player Killing and Player Killer Killing (ad infinitum) is Serious Business in The World.
The Vigilante alignment in City of Heroes is basically a Hero who kills instead of arrests.
Alex Mercer from Prototype in a way. He kills thousands of comparatively blameless soldiers to get to their leaders, the ones responsible for the outbreak of the disease ravaging Manhattan, and kill them. His reasoning, insofar as 'reason' factors into anything Alex does, seems to be that however many people he kills, his targets are responsible for both far more deaths and much uglier crimes. Of course, his conscience is still developing throughout the game, so at the beginning he was just a plain-old mass murderer.
- ...of guys who were trying to kill him for reasons he didn't understand at that point. (Unless he also killed civilians.)
Garrus Vakarian in the Mass Effect series between the first and second games. He became a vigilante in the in-universe Wretched Hive, hunting down all sorts of criminals, often administering poetic justice against the more particularly heinous ones. He brutalized a slaver, killed a drug dealer using the same addictive and deadly drug the dealer sold, killed a specialist in biological warfare with a virus, and killed a dangerous saboteur by causing a space suit malfunction.

Web Original

The Onion "Series Of Serial-Killer Killings Rocks Serial-Killer Community"
The Global Guardians PBEM Universe had The Confessor. The Confessor obsessively hunted down and murdered (usually in a very Karmic Death) other serial killers. When the heroes finally captured him, and one pointed out to the Confessor that he, himself, was a serial killer, The Confessor delivered the Hannibal Lecture to end all Hannibal Lectures.
- The Dove was also a Serial Killer Killer, but honestly thought he was the hero of his own story.

Other/General

Lots of protagonist vampires in fiction go through a Serial Killer Killer phase, deliberately preying upon only the guilty, before they swear off human victims entirely and embrace the Vampire Detective and/or Friendly Neighborhood Vampire tropes.

Real life

Christopher Scarver, an African American inmate who killed notorious serial killer Jeffery Dahmer as well as Jesse Anderson, both of whose crimes heavily affected black people. He claimed God told him to kill them.

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