He Knows Too Much

"Lucille: ...so there's no need to kill him.

Death Squad Leader: Yes there is, ma'am...once he's told us who else he's spoken to.

Lucille: What? [is machine-gunned by the Death Squad Leader]"

- Sin City

Somebody is involved in something dirty, or just did something nasty. Could be The Government, could be The Mafia or The Syndicate, could be General Ripper, a Corrupt Corporate Executive or an Ancient Conspiracy. No matter what bad guy was involved, somebody saw it all, heard it all, or somehow caught wind of what's going down (or what went down), and the bad guy in question has found out about the witness.

Since the witness now knows too much, the bad guy's entire scheme may come crashing down, so they aim to "silence" the witness in some manner, through bribery, blackmail, intimidation, or even murder.

A common variant is that the person who purportedly knows too much doesn't actually know anything at all (or at least, doesn't understand what knowledge they have) -- but the overly paranoid conspirators believe that they do, thus leading to their campaign of persecution. An excellent MacGuffin. This then leads to the ironic conclusion that if they'd just relaxed and left the person alone, they'd have succeeded, but since the hero eventually gets sick of being relentlessly persecuted, they then kick all kinds of ass in getting all up in the bad guys' business and completely scuttling their plans. If it isn't the hero getting persecuted, it will likely be someone the hero cares about, which will usually prompt either a bodyguard scenario as the hero tries to protect them against the bad guys, or a Roaring Rampage of Revenge if the loved one is killed.

The most common motive for bad guys to go after innocent people and the reason that the FBI has the Witness Protection Program.

See Revealing Coverup, Killed to Uphold The Masquerade and Have You Told Anyone Else. See also Leave No Witnesses for those situations where a roomful of people all Learn Too Much at once.

Anime & Manga
"'s computer screen: "You have witnessed too much.""
 * Sent in a message to in Gundam 00 just after he figured out the true agenda of , and just before , killing him in the process. Either sent by  or by

"Isaac: Do you know what shape this country is in?"
 * is also killed for knowing too much, but in a partial subversion
 * In Fullmetal Alchemist, learns about the Ancient Conspiracy, or at least a little more than the others know. We all know what happened to him....
 * In Brotherhood, so did Isaac McDougall. He also gets offed for his efforts.
 * It's not until quite a bit later in the story that the audience is shown exactly what he had uncovered. At first it seemed that he'd discovered little more than that there was an Ancient Conspiracy, and his knowledge shouldn't really have been a threat to them yet. But in fact, he'd also realized that, a fact that was the lynchpin of the entire conspiracy.


 * In episode 13 of Code Geass R2, gets shot to death for knowing that   The fact that  wanted to help  doesn't help, either.
 * In the first episodes of both the first and second season, Lelouch is about to be shot for having seen C.C., although
 * In Suzaku of the Counterattack, this is why had Marianne killed.
 * In an early episode of R2, Rolo murders a Britannian soldier who walks in on a conversation he's having with Viletta for hearing things he shouldn't. And all the guy heard was the word "Geass", which he couldn't possibly have even understood.
 * The scholars of Ohara got too deep in researching the Void Century (which is hinted at being a huge threat toward the World Government), and consequently the island, along with its inhabitants, was bombarded into oblivion by the World Government.
 * Played for comedy in Luffy/Ace's flashback. Luffy discovers where Ace and Sabo keep their treasure, and they state that they're going to kill him to keep him from telling, only to realize that neither of them have killed anyone before.
 * In Gundam Wing, all five Gundam Pilots were ordered to kill anybody who sees the Gundams or who discovers their identities. Heero takes it to an extreme, assuming that Relena knows too much without any actual evidence (she found him washed up on a beach in a spacesuit) and threatening to kill her for most of the series. In the second episode, Duo's a bit more forgiving, using a road flare to block Relena's view of Wing and Deathscythe and telling her that it's in her best interests to look away.
 * If someone possesses any evidence that Johan Liebert exists, that's enough to have Johan arrange for them to die. The exceptions to this are his sister (whom he considers his "other self"), General Wolf, and Dr. Tenma (the latter two saved his life).
 * Though, he's got his own plans for them. And they are just as evil.
 * Partially, this is what prompts to get the   to fall to his death in the Vampire Princess Miyu TV series. The fact that  really didn't help his case.
 * In the original manga of the Hentai series Bondage Fairies, main heroine Pfil is invited into the home of a trio of sisters while on a missing-stag-beetle case. While there, she accidentally stumbles upon
 * In Future GPX Cyber Formula, Osamu Sugo leaves the Missing Link F-1 team after knowing of his boss Smith's plans to use Asurada, the computer system created by the protagonist's father, as a weapon of mass destruction. He soon disguises himself as Knight Schumacher, in order to keep an eye on Hayato, as well as Smith and his men. During his racing duel with his rival Bootsvorz, Smith attacks Osamu from behind with his helicopter's missile for exposing said secret. Osamu gets better, but ends up injuring his eye.
 * Averted in Mai Hi ME. Akira is duty-bound to kill Takumi if he finds out she's a girl, but not only is unable to go through with it, but even saves his life by giving him his medicine.
 * In Naruto, Baki kills Hayate after he sees him meeting with Kabuto, learning that the Sand and Sound Villages are conspiring to destroy the Leaf Village.
 * This also happened to
 * Bell Matera of Striker S Sound Stage X, who was immediately forced to commit suicide the moment he figured out what the Mariage were seeking.
 * In Magical Record Lyrical Nanoha Force Marty gets his Head Asploded before he can tell Arnage and Veyron more about his employer.
 * In Detective Conan, Shinichi Kudo comes across a mysterious black man extorting from another man, but he fails to see the black man's partner, who sneaks on him and knocks him out cold with a lead pipe. The two tries to force feed him with a poison pill...but the pill only shrinks him into a body of a child and assumes the alias of Conan Edogawa.
 * In the Ski Lodge case,
 * The Non Serial Movie Phantom of Baker Street reveals
 * In the Luxury Liner case,

Comic Books

 * In Top Ten pop star and former science-hero sidekick Glenn Garland is killed because he was going to tell the media about the illegal and repulsive actions of a group which he used to be a part of. The Vigilante from Venus also expressed concern that they were going to try to kill her before she went to trial, for fear that she would incriminate them in her testimony.
 * This trope is what sets the whole plot of the first Sin City story "The Hard Goodbye" in motion, as Goldie is killed on orders from another character after she discovered his nasty secret habit. Another character dies after mistaking a death squad for honest cops and telling them everything.
 * Watchmen: Two people knew the same thing:
 * One is killed because he knows the details of a plan, even though he had no intention of revealing it; he had told Moloch, who simply didn't understand what he heard and would be no threat to its success even if he did understand it.
 * Open to interpretation. He MIGHT have told someone else, given time. Clearly he was fraying around the edges. The Big Bad being Dangerously Genre Savvy, he wasn't about to take the chance.
 * The other (not Moloch, by the way) is killed because he knows the plan and is going to reveal it. His murder may have been pointless, though, because he'd mailed his journal to the "New Frontiersman". Whether anyone of importance would believe what that Conspiracy Theorist tabloid reports is unclear, though.
 * This is essentially the reason that The Boys haven't been brutally butchered by ruthless 'superheroes' the Seven; the only reason they aren't dead is that they have even more damaging information on someone that The Seven fear more.

Fanfic

 * Magnificent Kamen in Sailor Nothing allows no witness to escape to reveal the Masquerade. The fact that  probably has something to do with that.

Film

 * Alfred Hitchcock was fond of this trope.
 * The Man Who Knew Too Much
 * North By Northwest
 * The 39 Steps, which was based on a book published during World War One, making this Older Than Radio.
 * Enemy of the State
 * Witness
 * The main action of the Brandon Lee movie Rapid Fire starts when Brandon's character witnesses a bad guy in the act of taking out a rival.
 * The Arnold Schwarzenegger movie Eraser.
 * Sister Act
 * Snakes On a Plane: The catalytic event for the titular serpents upon the aircraft.
 * Subverted slightly in Adventures in Babysitting -- one of the kids had taken a Playboy magazine that could get the crime boss put away, and the presumption the criminals made, of course, was that the kids had read it, plus the fact that they'd met at all meant they knew too much. The magazine was returned, and the crime boss' underling resigned, knocking him out along the way.
 * The Hong Kong action movie Sha Po Lang (known in the US as "Killzone") kicks off with the murder of a witness ordered by a Triad crimelord so that his testimony could not be given and the crimelord would be set free. This sets four cops against him, all with their own reasons for wanting to take him down.
 * In Lethal Weapon, Riggs is tortured by the bad guys to find out what he knows about their impending shipment, which is.
 * In Serenity, the Operative is sent after River because she picked up the secret of Miranda, which has been buried for twelve years, from the heads of top members of the Alliance's Parliament who came to the Academy to see her. The nature of the secret in question means that Parliament wants more than anything to keep it buried, even if it means River must die.
 * This is a perfect example of the variant, since River was totally crazy, so she couldn't have communicated what she knew to anybody who could do anything about it. The Operative coming after her and the crew compelled them to find out the truth.
 * The Man With One Red Shoe has the scheming deputy head of the CIA trying to frame the actual head, so that he can have his job. The head knows this, and lets slip that someone who can foil this plan very simply will be arriving at the airport. He then sends his aide to meet any random person at the airport, so that the deputy and his men will follow them and try to find out what he knows. The aide picks a rather head-in-the-clouds musician, who's wearing a pair of mismatched shoes because of a practical joke.
 * The Hero in Marathon Man is one of the actually-know-nothing kind.
 * It's pretty strongly implied that the "villain" of The Hunted was just acting in self defense, pursued by government assassins over his past black-ops experience.
 * justification for killing Leroy and Destiny in Mystery Team.
 * In Captain America the First Avenger, Col Phillips warns a captured Zola that Schmitt will see him this way.
 * In Drive a gangster uses a crew of low level criminals to rip of a Mafia boss with the intent to later kill them and take the money for himself. When the plan blows up in his face he and his partner decide that everyone involved Knows Too Much and must die so the Mafia never finds out who was really responsible. Ironically, the only survivor of the crew actually does not know anything and was about to leave town.
 * in Little Shop of Horrors, adapted from the stage musical. This trope did not apply to the original film.

Literature
""... You came to surprise a secret which no man in the world must penetrate—the secret of my whole existence. And you think that I am going to send you back to that world which must know me no more? Never! In retaining you, it is not you whom I guard—it is myself.""
 * In Duumvirate, anyone getting too close to certain truths is most commonly fed misinformation, with options for inflicted insanity, death, and acquisition.
 * An interesting variant in Animorphs - the heroes consider this during the David Trilogy, when the titular character turns against them. It makes them pretty uncomfortable, though in the end, they decide on an even more extreme solution.
 * The Mote in Gods Eye. After the three midshipmen crash land on Mote Prime, they wander around for a while and make a number of discoveries. The Motie decision makers decide to kill them to keep them from telling the other humans what they've found out.
 * And it plays out the other way, too. The humans are about to be captured, but they decide that they know too much about human technology to allow themselves to be taken prisoner--so they kill themselves.
 * In Girlfriend In A Coma, Karen, the titular character, believes this was why she went into her coma in the first place - she caught a glimpse of the future, and it wasn't pleasant.
 * In Dan Abnett's Gaunts Ghosts novel The Armour of Contempt, Ludd interrupts a No Holds Barred Beatdown intended to kill Merrt. The man ordering it assures him that the body would never be found here, and offers a bribe. Ludd says he could report it anyway, and the man admits he knows too much to live.
 * If you're a character in an Agatha Christie novel, don't ever try to blackmail a murderer if you want to live to the end of the novel. Amberiotis of One, Two, Buckle My Shoe and Louise Bourget of Death On the Nile found this out the hard way.
 * Joyce Reynolds, the 13-year-old schoolgirl from "Hallowe'en Party" (1969). She tries to use her knowledge of a murder to her benefit. Instead, the murderers have her killed, drowned in an apple-bobbing tub.
 * Another would-be blackmailer victims include Miss Blanche from Cat Among the Pigeons and Leopold from Hallowe'en Party. Other characters who "knew too much" include Donald Ross from Lord Edgware Dies and Mrs Upward from Mrs McGinty's Dead - as well as the principal murder victims in Three Act Tragedy, The Murder of Roger Ackroyd, Cards on the Table, Evil Under the Sun and A Caribbean Mystery.
 * An example of the second variant comes in Why Didn't They Ask Evans?. Bobby Jones witnessed the death of a man who stumbled and fell off a cliff, then tells a woman claiming to be the man's sister that his last words were "Why didn't they ask Evans?" This is enough to convince the conspirators that Bobby has to be killed, which is in turn enough to convince Bobby that the man was murdered, "Evans" knows something important, and he should investigate...
 * An example that's neither a blackmailer nor the principle murder victim is Salome Otterbourne in Death On the Nile, who witnesses the murder of Louise Bourget (see above), goes to tell Poirot, and gets to invoke His Name Is before being shot dead by the killer.
 * Syme in Nineteen Eighty Four lauds the principles behind Newspeak to Winston, explaining in great detail how it helps the Party perfect its control over the general populace. Despite being thoroughly loyal to the Party, he is eventually made an "unperson" due to this understanding. A fate Winston himself is able to see coming, predicting that Syme will be "Vaporized" several chapters before it actually happens.
 * Defied in Crown of Slaves. Some Solarians suggest offing, but are warned away by the fact that she is connected to knowledgeable and hardened killers (read: Victor Cachat, Anton Zilwicki and Jeremy X) who would take bloody vengeance, compounding the fact that the person who knows too much is the one they would go to for assassinations in the first place.
 * Twenty Thousand Leagues Under the Sea: This is the reason why Aronnax, Conseil and Ned Land will remain prisoners of the Nautilus and cannot come back to Civilization. Ever. (Captain Nemo lets them abandon the Nautilus and explore land, but is always in uncivilized shores). Captain Nemo explains:


 * Hanna gets when she recognizes A's number in Perfect

Live Action TV
"Dr. Culix: The boy on the bike? He's seen too much. Get him!"
 * River Tam from Firefly. We don't find out what it is that she knows until The Movie, but we know her handlers paraded her in front of many top-level politics and military leaders... without realizing that she is a mind reader, so the government definitely considers it too much. So much so that two government agents killed an entire building full of people for the crime of having heard her talk gibberish. While she was under arrest.
 * The Tomorrow People:

"Stringer Bell: "But there go a life that had to be snatched, (...) Twenty years above his fucking head. He'd flip, man! They got you, me, and Brianna! No fucking way, man! Hell, no! Now, I know you family, you loved that nigga, but you wanna talk that 'Blood is thicker than water' bullshit, you take that shit somewhere else, nigga! That motherfucker would've taken down the whole fucking show, starting with you, killer!""
 * Dollhouse: Rather than being "terminated" (one way or another), a "handler" who has been exposed for raping his assigned "doll" is offered a task more suited to his evident personality: eliminate a woman who has inadvertently learned too much about the Dollhouse from an FBI agent investigating the organization. Only it's
 * Used several times per season in 24, usually overlapping with You Have Outlived Your Usefulness.
 * UFO. A continuing duty of SHADO (Supreme Headquarters Alien Defence Organisation) is to suppress information on UFO incidents to prevent worldwide panic over the knowledge that beings from another world are harvesting human organs. Their methods include intimidation (ranging from beatings to government pressure on the employers of the witness), conscription into SHADO, amnesia pills, and even murder.
 * In the Mission: Impossible episode The Town, Phelps stumbles across a town of Russian spies and discovers the fact. They intend to make his death look like natural causes. Fortunately, he has four brilliant colleagues.
 * In the Doctor Who serial The Gunfighters, Johnny Ringo is in Tombstone looking for fellow Magnificent Bastard Doc Holliday, and the barkeep recognizes him, and threatens to go to the Earps.
 * This was the Doctor's justification for kidnapping Barbara and Ian. Couldn't have them knowing about the TARDIS and screwing up the timeline, after all. Because two humans would definitely be capable of reproducing such advanced technology after being inside of it for a few minutes.
 * Implied in an episode of Warehouse 13 after a teenage girl breaks into the Warehouse to coerce Artie to help her save her brother (as in, she knows about the Warehouse and now we have to do something about it). They seem like the kind of folks who consider death to be a (albeit viable) last resort, though.
 * She joins the operation as a sort of Sixth Ranger after the incident.
 * Something very similar happens in the first episode of Season 3- the warehouse agents run into ATF agent Steve Jinks on a mission, and their attempts to explain the situation away don't work because he's a Living Lie Detector. Since he's in on the secret and already a federal agent, he gets recruited.
 * Basically why who has learned that   in Battlestar Galactica Reimagined. Important to note that  is not in on this, and   when he finds out.
 * NCIS
 * The episode "Witness" was based on this motive. It included a medium Tear Jerker, as McGee narrowly failed to save the girl, who he'd hit it off with just the day before.
 * Arguably, this is also part of the reason that an airstrike is called upon a research vessel doing some questionable things out in the ocean in "Chimera". Fortunately, Gibbs and his team get away from the ship on a pirate's boat as the missiles are fired.
 * Several (main and minor) characters on The Wire have been killed off because of this trope. Whether they do or don't know too much, the results tend to be tragic.


 * Someone from Pretty Little Liars is in great danger in the Summer Finale of the show. A even sends a text to Emily which says 'She knew too much.'
 * In The X-Files, Mulder knows entirely too much right from the start, and then gathers more information along the way. When attempts to get him to come to the dark side fail, the Syndicate try to destroy him instead. Since Mulder is more valuable to them alive than dead, they tend to do terrible things to people he cares about. Arguably the worst was giving Scully an inoperable tumor that spelled a slow and painful death for a season.
 * An interesting variant occurs with John in Farscape because, while he knows that the Ancients put wormhole knowledge in his head, he isn't able to unlock that information himself for a few seasons. This leads to Scorpius chasing him across the universe and mind-probing him to figure out information that John is still trying to discover himself.
 * Cold Case
 * Part of the reason why the teacher in "True Calling" was murdered was because
 * The victim in "Blood On The Tracks", who wanted to confess to the cops about a crime he and several of his friends had been involved in that left another friend dead. With only one other person in the group agreeing with him, two of the others conspired to kill them both in order to ensure their silence.
 * Numb3rs
 * In the episode "Democracy", five people who had evidence of a voting fraud conspiracy are killed to uphold it. Unfortunately for the conspiracy, one of them managed to get the list of names to Charlie shortly before being killed, who determines that the odds of five random people dying in a given two week period are approximately a bajillion to one. Don orders a second autopsy on the one who gave Charlie the list, which confirms that she was murdered, and the attempt to tie up a few loose ends has now attracted the attention of the Federal Bureau of Investigation.
 * In "Disturbed", the episode after the credits opens with a murdered mailman who was last seen alive outside of a home where a woman was killed. He waves at the repairman entering the house. Nearly an aversion, since the mailman only possibly knew something.
 * Prison Break: Usually the reason Anyone Can Die.
 * The Shadow Line:
 * This is why Jonah Gabriel and his partner were shot before the events of the series, in that
 * is killed for investigating the above incident too closely, because his investigations could have led to what Gabriel and Delaney discovered.
 * And even invoked in episode 6, when
 * Happens from time to time in Twenty Four.
 * In Season 1,.
 * In Season 2, kills Reza just as he is about to clear his own name and find out who moved the funds for Syed Ali.
 * In Season 3, the reason why the Big Bad forces CTU to execute.
 * In Season 7, the villains kill after he is captured because he has information on them that he is using as blackmail to prevent being subject to You Have Failed Me and You Have Outlived Your Usefulness.
 * Parodied in The Black Adder. Edmund desperately tries to call off his plan to assassinate Dougal McAngus, who may have evidence proving his brother is an illegitimate child. When he tries to tell Percy that "He knows too much!" Percy, momentarily forgetting just why they are killing him (Dougal had stolen three titles from him), menacingly replies "That is why he must die!"

Music

 * "Jane Doe" by Within Temptation.

Tabletop Games

 * Somewhat predictably, Warhammer 40000 does this.

Theater

 * "He knows your life of crime! I think it's suppertime!"

Video Games

 * Max Payne: It turns out that the junkies who killed the title character's wife and baby girl
 * The protagonist in Arcanum is an example of the second type: all he knows is a gnome gave him a ring and told him to "Find the boy," but due to the conspiracy's repeated attempts to murder you, you eventually figure out their incredibly convoluted plans.
 * Fire Emblem 8: Some characters know well what the hell is going on in Grado (the Cleric Natasha  and the Shaman Knoll,  ). What does the Empire do? They frame * both* of them as traitors; Natasha has to flee from her homeland and Knoll is imprisoned and slated for execution. Subverted because, in-game, both * survive* and join Eirika's (in Natasha's case) or Ephraim's (in Knoll's) troupes.
 * Modern Warfare 2: After sending all of the TF 141 to both of Makarov's safehouses,  betrays them all because he suspected that they knew about his plans to make the U.S. the most powerful country in the world.
 * Hawke and Lash from Advance Wars Dual Strike are attacked when Hawke figures out the Big Bad's true motive for starting a war. Of course, the allied nations save them and they all fight the Big Bad together. Hm. Maybe the Big Bad was planning on getting rid of Hawke and Lash anyways, since Lash is caught up in Hawke's problem for almost no reason.
 * More or less. A few missions prior, Kindle outright states that they don't think particularly highly of Hawke OR Lash, and basically implies that they never really had much of a need for either of them. The aforementioned overhearing of plans basically just gives them a convenient excuse to actually do something about it.
 * In Halo 3: ODST, an NMPD officer on payroll is instructed to  and to eliminate any witnesses. Unfortunately for him, you are a witness. Let's see:   versus an elite UNSC shock trooper. Guess who wins...
 * In the Hitman series, this is pretty much the only reason 47 will kill an innocent (in canon), as his existence is already very volatile and he cannot afford letting anyone (other than a select few) know about him.
 * In Half Life, the Hazardous Environment Combat Unit are sent in to combat the aliens and kill all the scientists and security guards. Later on, Black Ops is sent in to kill the HECU survivors. Finally, the G-Man nukes the entire base at the end.
 * This sets up the plot of Robinson's Requiem: you and several other Robinsons have discovered that AWE banishes or kills any agents who have potentially contracted alien diseases or virii while exploring unknown planets. Unfortunately for you, AWE finds out that you've learned this Awful Truth and decides to send all of you to the Death World Zarathustra.
 * Before the Rank 8 fight in No More Heroes, Travis interrupts Shinobu's presentation before her class. She sighs, tells him to meet her outside, then draws her katana as he leaves. When they meet up, she's complaining about how hard it is to lead two lives... at which point Travis makes the connection: "You killed them, didn't you?"
 * Nine Hours Nine Persons Nine Doors:
 * In Sonny, this is the reason the ZPCI board the White November in the first game and destroy the village in the second game. In the former, Sonny kills the soldiers after they take out Louis, escaping on a lifeboat after recieving the MacGuffin. In the latter, Sonny and his crew fight the leader of the troops briefly before fleeing on an old train.
 * In Fallout New Vegas, if you convince Veronica to leave the Brotherhood of Steel for the Followers of the Apocalypse in her personal quest,
 * LA Noire has an example.
 * In Oddworld: Stranger's Wrath, after being
 * Blaz Blue Continuum Shift:
 * In Splinter Cell Conviction

Web Original

 * The Nostalgia Chick: In her review of Disney Princesses, The Chick guesses that The Princess and The Frog was only created because Disney needed to expand their Princess line. A ninja is sent to silence her.
 * The Nostalgia Critic: In his review of Full House, the Olsen Twins try and kill the Critic when he attempts to tell the world his new knowledge that they're evil.

Western Animation

 * Parodied in an episode of Jackie Chan Adventures, appropriately titled "The Chan Who Knew Too Much". A cabal of wizards repeatedly states that Jackie "knows too much" when he finds out any of the cabal's secrets... their name, their plans for Stonehenge, or even the location of the bathrooms in their secret lair. ("He's privy to our privies!") To make it even better, Jackie really didn't know anything at all about when he first accidentally dropped in on all of them - for all he knew, they could have just been wannabe satanists or something. They pretty told Jackie everything themselves, e.g showing him that they knew magic by trying to kill him with it.
 * Gloriously lampshaded when Jade claims to know their magic words, but can't cast their spells. The man gloats that she 'knows nothing' and Jackie cries out, "That's what I've been trying to tell you!"
 * In Batman Beyond: Said in its entirety by Inque about Terry McGinnis. It is also motive behind Derek Powers's murder of Terry's father, which prompted Terry to become Batman.
 * Played with in The Simpsons. Homer followed his coworkers to the Stonecutters' headquarters. He gets inside by falling through the skylight and is instantly surrounded. They declare that he has seen to much and must pay "the ultimate penalty". So they toss him out the front door.
 * In "New Kids on the Bleech", after Lisa discovers the Navy's conspiracy to recruit people via Subliminal Seduction, Lt. Smash says ominously "Well, know that you know, I'm afraid I can't let you leave." But Lisa already left.
 * In The Spectacular Spider Man, Punch Clock Villain Doctor Otto Octavius is entirely correct to fear that someone will discover his involvement in the creation of Supervillains made to antagonize Spider-Man. Indeed, Green Goblin engineers a Freak Lab Accident to silence the doctor for good. The catch? Octavius lives, though he's become much less meek and a lot more megalomaniacal. Unfortunately, the newly-christened Doctor Octopus assumes Spider-Man is the culprit.
 * Transformers Animated has, who was offed by the traitor for knowing to much.
 * Spoofed in the Looney Tunes short "Bugs and Thugs", where gangster Rocky decides that Bugs "knows too much" when he, among other things, correctly identifies Carson City as the capital of the state of Nevada.
 * The owl spirit in Avatar: The Last Airbender
 * Lampshaded in Scooby Doo and the Alien Invaders, where the villains say this as they prepare to kill Fred, Velma, and Daphne. Fred grumbles "That's always our problem..." in response.
 * In Futurama, a robotic toilet the crew is throwing away offers them "Happy Poopy Time" if they spare it. Fry simply says "You know too much."

Real Life

 * In 1942, the Russians arranged an assassination of Von Papen, the German ambassador to Turkey. They gave the would-be assassin a package which they said was a smoke bomb to cover his escape. The assassin decided that attacking under cover of the smoke was a better plan and so set it off early: it exploded killing the assassin but leaving von Papen unharmed except for the gore splattered on his suit. The Russians of course intended him to do this after shooting the target, thus preventing him from telling who was the guilty party.
 * Undoubtedly inspired by this incident, Ian Fleming had two SMERSH assassins fall victim to this trick in his first James Bond novel Casino Royale.
 * This is the reason why, in a lot of urban neighborhoods, people won't talk to the police about crimes they saw, and is also the basis of the "snitches get stitches" saying.

Urban Legends

 * The Home Guard Auxilary Units were stay-behind units trained in the event of a German invasion of Britain during World War II. Each patrol consisted of local men recommended by their Chief Constable. They had secret orders only to be opened in the event of an invasion. One man opened his anyway and found to his shock that his first mission was to assassinate the Chief Constable, as he was the only man who knew the identity of the unit members.
 * In a joke, a KGB agent is questioned by his superior: "How much is 2 plus 2?" - "Four." - "And how much is 3 times 5?" - "Fifteen." - "And how much is the square root of 9?" - "Three." At the moment, the superior draws his gun and shoots the agent. "Comrades, I had to neutralize him, he knew too much!"
 * Damn. Who knows what they'd have done to him if he answered "&plusmn;3" for the last question?
 * He would have been wrong, because that only applies when solving equations.
 * Thus he'd probably be shot for not knowing enough.