A character (usually the main character, but sometimes sharing equal billing) is chasing or being chased by a killer or monster. A lot of people end up dead, but not this character, and in the end we find out why: the 'hero' was the killer all along.
This can go one of two ways:
Secretive Killer: The 'hero' is a willing killer or monster who has been trying to shift blame or tie up loose ends.
Amnesiac Killer: The 'hero' somehow blacked out the murder(s) and is as surprised and horrified as the audience to discover the truth.
The Amnesiac Killer is the more common variation, primarily because it's easier to write, as the audience is taken through the same reveal as the character himself. The Secretive Killer is particularely hard to pull off with the main protagonist (a minor protagonist is more commonly used for this reason), since the audience is always told the story from their perspective. If the events are shown directly, maintaining the twist right until the end therefore runs the risk of either Out Of Character Moments or losing focus on the character to conceil their real identity. A reliable method is to have the events (re)told through an Unreliable Narrator.
In many cases, the Amnesiac Killer is essentially a Memory Gambit told In Medias Res. A Split Personality is common. In Speculative Fiction, this may be due to a Superpowered Evil Side or Enemy Within.
When handled with care, it can be a powerful Twist Ending. When tacked on just for the sake of surprising the audience, however, it is likely that key events in the story will become illogical, or in the worst case, physicallyimpossible.
Compare Tomato in the Mirror, in which the character finds out some other truth about who or what they are.
Not to be confused with The Killer Inside Me.
Huge spoilers ahead of course!
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Examples of Secretive Killer:
Perfect Stranger - Halle Berry's character murdered her 'friend' (who was blackmailing her) and successfully pins the blame on the innocent, if slimy Bruce Willis. Oh and she murders again when another friend tries to blackmail her about setting up Willis. This one is pretty much the result of last-minute Executive Meddling to decide the identity of the killer.
The protagonist of Dario Argento's Tenebre is revealed to have killed the murderer — an obsessed fan of his — halfway through the movie and then used it as a cover for a murder spree of his own.
One of the earliest movie examples: Forbidden Planet, via "Monsters from the Id." Though in this case the murderer is established as not being willing except on the level of the Id. He just fails to inform the other characters that his subconscious is creating monsters running around killing people.
Along Came A Spider - Monica Potter's character is the mastermind behind the kidnapping, who orchestrates the murders she is supposed to be investigating.
The (fake) FBI agents played by Bill Pullman and Julia Ormond in Surveillance.
The protagonists of A Perfect Getaway, are eventually revealed not to be in fear of the killers, but to actually be the killers. The other couples who are presented as potentially the killers are either their next targets, or stooges to pin the murders on.
The protagonist of Beyond A Reasonable Doubt murdered a woman, covered it up, framed himself for murdering her (it made sense in context), got a mistrial declared, and finally was caught in the end.
In the French film Vidocq, the title character is a detective who disappears chasing the mysterious murderer known as the Alchemist. Vidocq's biographer Etienne takes up his investigation. Eventually, it turns out Etienne is the Alchemist himself, who tries to find and eliminate all clues leading to him that Vidocq left.
Aaron Stampler in Primal Fear, who reveals at the very end of the movie that his innocent persona was all a lie.
Martin Vail: So there never... there never was a Roy.
The Uninvited qualifies as this. In addition to being a Faux Horror Film, it turns out that Anna murdered every person that died. She doesn't remember anything she's done until a massive flashback scene. It also comes with The Reveal that her sister Alex has been Dead All Along.
Although they never spell it out for you, this is subtly implied to be the case in The Cabinet of Dr. Caligari. The film is mostly presented as a tale-within-a-tale, told by Francis, about how he exposed the wicked Dr. Caligari and his sleepwalking minion Cesare as the murderers of Francis' best friend. At the end, back in the Framing Story, we learn that Francis is an insane asylum run by the man he thought was Caligari, and that the sleepwalker (who never died, as he did in Francis' story) is a fellow inmate. The one person missing is the murdered best friend, Allan, and there are clues throughout the movie that Francis killed him because he saw him as a romantic rival.
The detective-narrator of William Weld's Mackerel by Moonlight. Well, sort of. He's actually innocent of the murder he's accused of, but the twist ending reveals that he's secretly the Russian mole that the villain blamed the murders on.
James Patterson's novel The Lake House does this; the beginning of the book says that the events are being told by the characters and may be inaccurate, but you're likely to ignore it until The Reveal.
Cat and Mouse by James Patterson. This case combines parts 1 and a bit of a part 2 as well.
James Patterson's Beach Road. This trope was so skillfully done that if you aren't paying close attention, the end will throw you for a massive loop.
Used in season four of Angel: Cordelia was the Big Bad masterminding the events of the first two thirds of the season.
In one episode of Porridge, an elderly man in prison for murdering his wife years ago repeatedly protests his innocence. After eventually being granted a full pardon, he reveals he knows exactly what happened to the actual murderer: He killed him before being arrested for the wrong murder.
In the Superstar episode of Buffy the Vampire Slayer there is a monster running around attacking people. It turns out that it is a side effect of the spell Jonathan cast to become the Uber Cool Guy. Slightly subverted in that it was a separate entitiy.
The song "Buenas Tardes Amigo" by Ween appears to be about a man seeking revenge against his brother's killer, but in the end it is revealed that he did the deed out of jealousy, pinned the blame on someone else who promptly fled, and is about to seal the deal by killing the man he framed.
In Death Note, Light Yagami, who repeatedly insists that he is not the supernatural killer Kira, and does not remember being Kira, is Kira. Note however that this only applies during his Memory Gambit during the Yotsuba arc.
In the manga-only chapter Onisarashi-hen, viewpoint character Natsumi Kimiyoshi is responsible for the deaths of her family members, though she doesn't realize until the end due to Trauma Induced Amnesia.
In the first arc Onikakushi-hen, the main character Keiichi realizes that people around him are acting crazy and people are dying... He gets increasingly afraid, and kills his two best friends under self-defense. It wasn't. He was just being paranoid and delusional.
In Monster Inspector Lunge assumes this is Tenma's problem for pretty much the entire story because all the evidence he can find says Tenma is the killer.
Most well-known in Cluedo/Clue, since the murderer could be any of the six Player Characters. Including yours, and you don't know if you did it or not (unless you've got your own card in your hand, or seen it in someone else's). And, as amusingly pointed out in Murphy's Rules, if you deduce that you are the killer, you win the game by denouncing yourself.
Inverted in Alan Moore's Greyshirt comic, when a man finds himself with a bloody hammer and a dead woman, and no memory of either. He reads in the newspaper that the Hammer Killer has murdered eight people and flees the police as it must be him. But when Greyshirt and the police find him, they tell him he was the next victim: the Hammer Killer slipped while attacking him and broke her skull. Unfortunately, he has killed someone who tried to stop him, thinking he was a multiple murderer anyway.
An issue of Cable & Deadpool featured our favorite merc investigating a murder in Providence, his buddy Cable's brand new Utopia, only to have the investigation end with a two-page spread of Deadpool saying, "Now the only question is... why did I kill this man?!" It eventually turned out Deadpool's healing factor was interpreting traumatic memories as damage and healing them over, leading to periodic blackouts.
In Batman Two Faces Elseworlds, Bruce Wayne used a potion to transform into Batman who is the embodiment of all his good qualities. A mysterious killer who laughs started appearing and killing prostitutes and Batman chased after him. Only after investigation, it was discovered that he was the Laughing Killer who was the embodiment of all his bad characteristics.
In The Boys, the Homelander believed he was suffering from this after receiving photos of himself performing heinous acts like baby eating. It's really his clone Black Noir trying to drive him insane so Noir can receive the order to kill him.
The Number 23 - Jim Carrey's character murdered his cheating girlfriend and suffered a guilt induced nervous breakdown causing him to forget the whole affair. The titular book was written by Jim's character (who wrote the rough draft), and his doctor (who fictionalized the story).
The film version of Secret Window - John Turturro's character does not exist; he is a schizophrenic hallucination undergone by Johnny Depp to commit acts (murder, arson) Depp could never consciously bring himself to do.
Shrooms - There are no ghosts or wild killers and Lindsey Haun (probably) cannot see the future; the mushrooms have driven her violently insane and she is the one who kills everyone.
Fight Club: Toward the end of both the novel and the movie, Tyler Durden and the unnamed protagonist are revealed as physically being the same person.
In Dragonball Evolution, Goku finds out that the evil Oozaru, which he's spent most of the movie on a mission to defeat, is actually him. Somehow. Even though the Oozaru is supposed to be thousands of years old and Goku himself is only a teenager. It's not really explained.
Identity - Our "hero" turns out to be one of a serial killer's alternate personalities, and all the deaths in the movie are of various personalities inside the killer's head.
Haute Tension (also known as Switchblade Romance in the UK and High Tension in the U.S.) - The insane truck driver who ties up Marie's friend Alex and murders Alex's family... turns out the be Marie herself; the insane truck driver is her alternate personality. Alex, the film's true heroine, is abducted by Marie because of a suppressed lesbian attraction.
Spoofed in Adaptation: Donald's hackneyed script "The Three" has the twist that the killer, the detective, and the victim are all the same person. Charlie complains that it makes no sense, but it's a smash hit anyway.
This premise was made into a proper film called "Thr3e." The book it's based on is very popular among the (surprisingly large) Christian suspense fiction community.
Angel Heart detective Harry Angel is hired by Louis Cyphre to track down someone who skipped out on a deal with him, and is serious about it enough to kill to cover his tracks. After travelling to New Orleans to investigate and being framed for several murders Harry comes to the horrid revelation that he is the missing person, had sold his soul to the devil, who has set the whole thing up so that he will be executed by electric chair and then go to Hell, thus fulfilling his debt.
Memento. The protagonist has frequent short term memory loss, and is trying to find the man who killed his wife. In the end, he accidentally killed his wife through an insulin overdose, and chose to preserve his sanity by rehearsing a story that it all happened to someone else, called Sammy Jankis. So now he chases criminals in revenge for an act he himself committed. Maybe.
In Triangle, Jess, who winds up killing all other characters (including herself) in doppelganger form, and is eventually revealed to be continually reliving the same sequence of events because she's a) insane, b) dead, or c) both.
In the 80s TV movie Blackout, Keith Carradine's character is in a car accident with another man, who dies. Keith loses his memory and is hideously disfigured. One of the two men is/was a serial killer, but no one, including Keith, knows which one. Until Keith starts having blackouts, dressing up in bondage gear, and trying to axe-murder his wife.
At the end of Blair Witch 2: Book of Shadows it is revealed that the three remaining main characters have been killing off their friends over the course of the movie, who they were hallucinating as evil people.
The VERY historically inaccurate film of "The Boston Strangler" says this is the case with the killer, Albert De Salvo, and much of his time in custody is spent trying to get him to realise that there's another personality inside the gentle family man. This proves difficult, because he doesn't actually remember blacking out during the killings, and has false memories of what he was doing at the time. It should be noted that the real Albert De Salvo was never even suspected, much less diagnosed, with having any sort of Multiple Personality Disorder
Homaged in the Wallace and Gromit film The Curse of the Were-Rabbit: The duo spend the first half of it trying to find a monstrous rabbit-creature that's destroying everyone's prized gardens. Turns out it's Wallace after a science mishap has caused him to transform into said creature during nightfall.
This is a popular interpretation of The Descent, with Sarah as the killer.
Played with on a less grander scale in A Nightmare on Elm Street Part 2: Freddy's Revenge. Freddy kills Coach Schneider. The camera than pans back to Jesse looking at the dead body, covered in blood and wearing Freddy's killing glove. When Jesse sees the evidence and realizes that he is the killer, he freaks out. In a later scene, Freddy bursts out of Jesse's body and kills Jesse's friend Grady. Pan back to Jesse again covered in blood and wearing the glove. He curses at Freddy, who is still standing in the room, but when he throws the glove at Freddy it breaks the doorroom mirror that Jesse saw Freddy in, revealing that Jesse was only looking at a reflection of himself.
Stephen King's novella "Secret Window, Secret Garden". Later the Johnny Depp film adaptation.
The novella may have been a case of Maybe Magic, Maybe Mundane. The mississippi writer is a real person who died prior to theevents in the book. His ghost may have done all the things the main writer did, or may have driven him to them, OR may not have been a ghost at all.
Interestingly, the Jim Thompson novel The Killer Inside Me isn't an example of this trope, as it's told from the first-person perspective of a character who knowingly and admittedly is committing the murders in the story, and isn't hallucinating or hiding anything. Or is he?
The Nothing Man by Jim Thompson: Inverted. It turns out the protagonist, who had thought he was killing people the entire time, invented all of it; every one of his "victims" either died of an accident, committed suicide, or survived. This probably wasn't the original ending he had in mind.
Mariastella Cosentino in The Scent of the Night- throughout the book everyone is looking for Gargano, who stole a lot of money from some very angry people. Mariastella was in love with him and when he came to her for refuge, she shot him and blacked it out completely, so to her Gargano was still alive and missing, not dead and wrapped in plastic on her spare room bed.
The Erast Fandorin story Leviathan features an English aristocrat who accidentally killed his wife through dangerous driving, went insane as a result and has internalised the story: he recounts it as though it happened to someone else and turns violent when confronted with the truth. This isn't a major part of the story and is not a revelation to the other characters, but is to the reader.
Oedipus Rex is a classic example of this trope.
At the end of 'Monster' by Diana Hoh, the protagonist disovers that she is in fact the monster who has been attacking her fellow students, due to a science experiment that went wrong, causing her to transform into a monster and have no recollection of the attacks. A very surprising twist, as she never even suspected herself. Also a theme in another of Diane Hoh's books, 'The Night Walker,' in which Quinn suspects that she is attacking people while sleepwalking. It turns out she isn't, the real culprit is trying to frame her for it.
Ironically, the Buffy the Vampire Slayer episode "The Killer in Me" is not an example of this trope. The earlier episode "Sleeper" however has Spike as a Manchurian Agent under the control of the First, killing and then having no memory of having done so.
The Angel episode "Somnambulist" at first appears to be an example of this trope, but it turns out that Angel is having dreams as a result of his connection to one of his vampire progeny, who is the actual murderer. This does make Angel indirectly responsible.
Davis of Smallville, who turns out to be the human form of General Zod's pet project, aka Doomsday.
One murderer in Criminal Minds didn't seem to realize he was a killer until the BAU themselves showed him proof. In this case, though, it's made abundantly clear to the viewers that he's the killer from very early on, so it's only a shock to him. This leads a rather bizarre and intense interrogation scene where the BAU keep showing him more and more evidence that he did it and he keeps refusing to believe it...until it all comes back to him.
In the Beyond Belief Fact Or Fiction? segment "Malibu Cop," the titular detective discovers that he himself is the murderer he is searching for. He committed the crime while sleepwalking. Supposedly this story was "Fact."
Subverted in an episode of Law & Order where a sleepwalking cop (who has a rather mean personality while doing so) is the suspect, but it's quickly established that he wouldn't have been able to commit the murders since his sleepwalking self couldn't use a gun. It turns out that his neighbour had been manipulating him (and preventing him getting treatment) so she could use him as a fall guy.
In season 6 of Dexter Professor Gellar is a split personality of Travis after he killed the real one. This is one of the less effective uses of the trope, as the stretched-out one-season story arcs of Dexter meant that much of the audience already saw this coming a mile off when the signs were becoming too obvious.
The TV movie In The Shadow Of Evil is about a cop who develops amnesia while on the case of a serial killer whose pattern indicates he will kill again in a month. Towards the end of the month he has regained enough of his memory to realise he's investigating himself and switches from Amnesiac Killer to Secretive Killer.
In the episode "Chimera", Ellen Adderly was horrified as anybody with the murders of first her friend and later her rival in their community. At one point she believed she was about to be attacked by the stringy-haired creature she saw in the mirror. It was found out that she suffers from Split Personality and her aggression was how she dealt with her husband's cheating on her.
The copycat killer in "Grotesque" was a leading investigator of the original case. As a part of the team he was chasing himself. Did he know? Maybe, as he specifically requested Mulder who he believed is the only one who could crack the case. However, he seems genuinely appalled and distrustful when Mulder reveals to him that it is him who they are chasing. In addition, as the original Serial Killer, he claims that he was possessed by a demonic force. A classic Maybe Magic, Maybe Mundane of The X-Files.
In Once Upon a Time, Little Red Riding hood was told to stay inside whenever the full moon was out, because a gigantic wolf was stalking around the village. It turns out she IS the werewolf, and her Granny was trying to protect the other villagers by keeping her inside. Red Riding Hood of course had no idea it was her, and often snuck out anyways, causing gruesome murders every so often.
Krayzie Bone's song Don't Know Why. Krayzie essentially blanks out after killing his ex employer but don't remember doing it. He wakes up in a grave yard covered in blood. Of course things get worse for him when cops show up at his front door more or less confirming his fear..
Arnold Schoenberg's song-cycle/single-role opera Erwartung (Expectation) — In a forest, an amnesic woman looks desperately for her lover. She finds his bloodied body and cries out for help. Her memory gradually falls into place — she has killed the man for his infidelity.
The Suffering: Ties that Bind: At the end of the game, it is revealed that Blackmore is really Torque's alter-ego.
In Max Payne 2 The Fall Of Max Payne, the show within a gameAddress Unknown ends like this: The protagonist, who spent the entire show looking for the murderer of his family, a man named John Mirra, realizes that he was the killer after he sees the killer's face staring back at him in a bathroom mirror. This, incidentally, may mean the whole purpose of the show was to set up a truly terrible pun*
by some mooks you hear discussing the show: he solves the mystery in the bathroom (as in, the john) by looking into the mirror... Also a Tomato in the Mirror.
In the indie adventure game Downfall (2009), the protagonist, Joe, realizes that the entire game has been subterfuge for his shotgun-toting rampage through his own apartment building. In true Silent Hill fashion, all of the monsters were innocent tenants, and Joe's damsel in distress, Ivy, is long dead. A police detective pops up in the game's finale to inform us that, no, we're not really in a hotel, and "Ivy" is just some lookalike whom Joe kidnapped and killed.
Inverted in Tsukihime, where Shiki sees himself walking around the city at night and killing people when he should be sleeping, and fears that this trope is in affect. Actually, he's watching the experiences of the real killer SHIKI, with whom he shares a spiritual link. Although he really DOES have a killing impulse and a killer in him, but it only targets non-humans.
In Silent Hill 2, James goes to the town of Silent Hill in order to determine whether or not his wife — who died three years ago of an unnamed illness — is still alive after receiving a letter from her. In reality, he smothered her with a pillow days before, may have her in the trunk of his car, and repressed the memory of it ever happening. The town of Silent Hill forces him to realize this truth. Depending on the endings, it's either this or a Mercy Kill
Part of the backstory of Siegfried in the Soul Series. He swears to avenge his father's murder. Guess who did it..
Patrick Star parodies this trope. He was trying to find who made a clam cry, and finally found it was... himself. Complete with, "It's the perfect crime!" Spongebob did it, though. Except he didn't. It Makes Sense in Context.
Parodied in the episode where Spongebob becomes a hall monitor: he begins hunting down a maniac that has been terrorizing the town, unaware that the "maniac" actually refers to him doing his antics of the episode. Leads to a bizarre scene where Patrick is using a walky-talky to report on the maniac's occasion and somehow, Spongebob just can't run away from him!