I'd Tell You, But Then I'd Have To Kill You

"Doctor: Oh, Mr. Holmes, I would love to tell you, but then, of course, I'd have to kill you. Sherlock: That would be tremendously ambitious of you."

- Sherlock, "The Hounds of Baskerville"

Tell me anyway!

Often heard in settings related to espionage and high security levels, the phrase "I'd tell you, but then I'd have to kill you" is itself probably a Dead Horse Trope by this point - whether for a serious straight use or not.

Nowadays used much more tongue in cheek to poke fun at stern security protocols and classification, it has started to show signs of decay since it's hard to pass off a Stock Phrase as an original joke. Instead it's more common to get it as one characters joke to another, perhaps to demonstrate that character's lack of regard to the rigmarole of secrecy or mockery of the other character's clearance/understanding.

A common modern comedy subversion is to have one character say the phrase to another with the response "So it's classified?" they then answer "no, I just feel like killing you."

Compare Just Between You and Me, in which a Big Bad tells the hero something they shouldn't before they kill them.

See the Supercut Mashup trailer of this line being said a bunch in films.

Anime&Manga

 * Japanese speakers don't have this Stock Phrase, but people like Heero Yuy skip straight to "omae wo korosu" when people like Relena Dorlian find out too much.

Film
"Maverick: We happened to see a MiG 28 do a 4g negative dive. Charlie: Where did you see this? Maverick: It's classified. I could tell you, but then I'd have to kill you."
 * Top Gun

"Paul Allen: Well, Halberstram, I could tell you... but then I'd have to kill ya."
 * American Psycho

"Jamie: I'd tell you, but then I'd have to kill you."
 * A Walk to Remember

"Mick Taylor: I could tell you. But then I'd have to kill you."
 * Wolf Creek

": "Blessed are those who do not know, for they shall live a while longer.""
 * Chris Farley's character uses this line on a little boy on a plane in Beverly Hills Ninja and makes the kid cry.
 * A variant in The Long Kiss Goodnight: "If you want me to talk in front of him, you may be asked to kill him later."
 * In the 2009 adaption of The Picture of Dorian Gray, Dorian quips this in response to being asked his secret of looking young.
 * Agent One uses this is Johnny English. When Johnny himself tries it later, he gets the response "I'd like to see you try."
 * In one of the The Witcher books, Geralt finds himself present during a sorcerous putsch, and the putchists capture him. Asking the leader what's going on, he is told sarcastically,

Literature

 * Discworld: Vetinari (then a young assassin) in Night Watch -- "I'd tell you but then I'd have to find someone to pay me to kill you."
 * The first title of The Gallagher Girls series is I'd Tell You That I Love You But Then I'd Have to Kill You.
 * says this in American Gods - the Fat Kid naturally assumes he's kidding. When the kid replies that yes, he really does want to know... he tells him. And then he kills him.
 * "Were I to tell you matters pertaining to the internal politics of the House of the Dragon I should only weary you. And I would then have to kill you for knowing. So my thought was not to trouble you with such information."
 * From The Three Musketeers, : "Because I am believed to be dead, and have reasons for wishing nobody to know I am living; so that I shall be obliged to kill you to prevent my secret from roaming over the fields."
 * An even more perfect example, from the sequel The Vicomte de Bragelonne: "It's a state secret," replied d'Artagnan, bluntly: "and as you know that, according to the king's orders, it is under the penalty of death any one should penetrate it, I will, if you like, allow you to read it and have you shot immediately afterwards." (The secret in question is the identity of the Man in the Iron Mask.)

Live Action TV
"Weyoun: "I have news." Damar: "Well?" Weyoun: "Your friend doesn't want to hear this." Damar: "And why doesn't my friend want to hear this?" Weyoun: "Because if she did I would be forced to have her executed." [Damar's lady friend leaves] Weyoun [somewhat snarkily]: "What a pleasant woman.""
 * Star Trek: Deep Space Nine: A scene between Weyoun, Damar, and Damar's lady friend, in the war room.

"Dr. Nigel Townsend: Counterintelligence for the Royal Navy. I'd tell you more, but then I'd have to kill you."
 * The West Wing: Season 7- Ep. 10 Running Mates (00:23:42 on the time bar), communications officer Will Bailey applauds deputy national security advisor Kate Harper for not using the line. (He also mentions how alluring he finds the fact that she actually could kill him with her bare hands...)
 * Crossing Jordan episode "Out of Sight"

"Joe Garrelli: There is a secret word that, when uttered, forces the judge to rule in your favor, then go to a secret location to paddle themselves in a secret ceremony. Dave Nelson: What is it? Joe Garrelli: I'd tell you, but then I'd have to kill you. Dave Nelson: All right, but it better be good."
 * News Radio episode "Rose Bowl"

"Shawn Spencer: Or perhaps you're referring to my license to kill. Revoked -- problems at the Kazakhstan border. I'd give you the details, but then I'd have to kill you, which I can't do because my license to kill was revoked."
 * Psych episode "Poker? I Barely Know Her"

"Stephen Colbert: I could tell you, but then they'd have to kill me."
 * Stargate SG-1 uses a variant where an offhand comment is made about O'Neill's time of service and the possibility of writing a book about it. Due to much of his duty even before joining the SGC consisted of top secret operations, he jokes that he'd have to kill anyone who read it.
 * The Colbert Report is going to the Persian Gulf, but exactly when or where it will be has been sworn to secrecy by the Pentagon.

"Alan: What happened to MIs 1, 2, 3 and 4? Stephen Fry: Well it's very interesting, I could tell you, but then I'd have to eat myself."
 * QI:

"Jenny: If we told you more about the show, we'd have to kill you."
 * One Alias promotional video spoofs this with Jennifer Garner addressing the audience.

"Secret Service Agent: Who's Mrs. Frederick? Artie: I'd tell you, but then he'd have to kill you (pointing at Mrs. Frederick's bodyguard, who has come to pick Artie up)"
 * In the "Implosion" episode of Warehouse 13:

"Kate: Steve Adler is your fraternity brother? Tony: I'd show you the secret handshake, but then I'd have to kill you."
 * In The Sentinel, when Jim and Blair go on a classified mission, Blair goes into a discussion about how police departments indoctrinate their members like a cult—Jim jokingly agrees, and says that, now that he's told Blair about it, he "will be required to kill you."
 * In Merlin, Merlin tries to lampshade this by saying "Right, 'cause if you do you'll have to kill me I suppose," but Arthur resopnds with "Immediately and without hesitation."
 * On Caprica, a character uses a Tauron phrase that is subtitled as "If I revealed the details, I'd have to return you to the soil."
 * On NCIS, when Kate finds out that Tony knows her new boyfriend:

"Xander: [On why the life sucking mummy won't tell him why she's crying]: Hey, I know why you can't tell me. It's a secret, right? And if you told me, you'd have to kill me. Ampata: [cries louder] Xander: Oh! That was a bad joke, and the delivery was off, too. I’m sorry."
 * Played with in one episode of CSI: NY. During the Cold Open Adam is talking to a girl via webcam, using some fictional form of Chatroulette. He asks her something - can't remember quite what - and she responds with this phrase...at which point
 * In Jake 2.0, Jake's younger brother steals Jake's wallet and goes to a bar. A girl notices the NSA ID, so the brother starts pretending to be a spy. He starts saying this phrase, and she finishes it off, annoyed at the cliche. He corrects her that he'd have to sleep with her, "but if that's your thing..."
 * Buffy the Vampire Slayer: from the episode Inca Mummy Girl:

"Daisy: Actually, what is your job, Mike? Mike: (Sincerely) If I told you that I;d have to kill you. Daisy: What, is is a secret? Mike: No."
 * Spaced goes for the 'modern comedy subversion' option:

Music

 * MF DOOM in the song "Superfriends": "I don't think you wanna know, even if you still do, and want me to, I'll tell you, but then I'll have to kill you."
 * "Weird Al" Yankovic's ode to Espionage Tropes, "Party in the CIA": "Burn that microfilm, buddy, will you, / I'd tell you why but then I'd have to kill you."

Newspaper Comics

 * Said almost verbatim in a Dilbert strip.

Real Life

 * In America's Got Talent, after the amazing presentation by quick change artists David and Dania, Brandy Norwood asks David "How do you do that?", to which an exhausted David replies: "We could tell you, and then we'd have to make you disappear".

Tabletop Games

 * In Paranoia one suggested variant on "That information is outside your security clearance" is... well, this trope.

Video Games

 * In the second Gabriel Knight Adventure Game, during the hunting club reunion, if Gabriel asks Von Zell what he is talking about with Klingmann, Von Zell replies "We could tell you, Herr Knight, but then we'd have to kill you."
 * Banjo-Kazooie: Nuts and Bolts: if you pay Bottles the Mole 6,000 musical notes (the highest price for any single item in the game) to tell you the "truth" about the infamous Stop 'n' Swop, he responds with "I could tell you, but then I'd have to kill you, and we can't show that in a game with this rating. Thanks for the notes!"
 * Blair, in Wing Commander IV when he comes aboard the TCS Lexington, uses this line to poke fun at Maniac.

Web Comics
"GOD. DAAAAAAMMNN."
 * Seen in Sweet Bro and Hella Jeff:


 * Seen in Xkcd where one character asks another what he's up to and receives the reply "If I told you, I'd have to kill you. I mean, sooner."
 * Girl Genius combined this with "The Easy Way or the Hard Way" here.
 * Mocked by Bug.
 * In one El Goonish Shive strip Justin worries that Mr Verres may have to kill him over things he's already told him.

Web Original

 * Chester A. Bum admits that he's Super-Bum. He then says that since he's told you, he'll have to kill you.
 * Good news is he's just joking- he actually makes the viewer impotent.

Western Animation

 * Referenced in Kim Possible. A soldier working on a top secret project blurts out "Neutronalizer" during Wade's briefing. Mr. Dr. Possible asks "This isn't one of those, 'I'd tell you but then I'd have to kill you' sort of things is it?'

''You now know too much about this trope. *BANG*''