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 character's 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.
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Japanese speakers don't have this Stock Phrase, but people like Heero Yuy skip straight to "omae wo korosu" (literally "I will kill you") when people like Relena Dorlian find out too much.
In the New 52 version of Resurrection Man, Mitch asks a demon what he's planning, and gets the reply "I could tell you that, but then I'd have to kill you. Repeatedly."
In Tempest of the Fae a goblin operating the mine cart Harry, Hermione and Luna ride in says "If I told you, you'd have to kill me." The stated rationale is that while it isn't technically illegal for them to be in possession of that particular information, offering it freely would count as treason.
In Bleeding Clown Harry and Snape communicate via two-way journal over the summer. Harry asks where Snape stays when he's not at Hogwarts and the reply is "In one of the Snape family manors, a highly unplottable location. If I told you, I'd have to kill you."
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,
Philippa Eilhart: "Blessed are those who do not know, for they shall live a while longer."
Played with in Men In Black: J tells Laura about the MIB. Then says "I'm sorry, I'm gonna have to...", to which Laura responds "Kill me." But just a little flash with the neuralyzer, and everything goes back to the way it was.
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.
"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, Athos: "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.)
The Honourable Schoolboy by John Le Carré - Ricardo explains his monologuing problem to Jerry: "You confuse me, Voltaire. If I tell you too much, I have to shoot you. I'm a very talkative person, you follow me? I get lonely up here, it is my disposition always to be lonely. I like a guy, I talk to him, then I regret myself. I remember my business commitments, follow me?"
Paladin of Shadows: Used as a joke by Mike, when asked by a beer distribution company representative how Mike had earned the money he was fronting for the Mountain Tiger Beer brewery.
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 calmly walks out]
Weyoun [somewhat snarkily]: "What a pleasant woman."
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...)
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.
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.
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.
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 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 responds 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:
Kate: Steve Adler is your fraternity brother?
Tony: I'd show you the secret handshake, but then I'd have to kill you.
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 Chat Roulette. He asks her something - can't remember quite what - and she responds with this phrase...at which point the murderer sneaks up behind her and kills her right in front of Adam, kicking off the Mystery of the Week.
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..."
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.
Spaced goes for the 'modern comedy subversion' option:
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?
In one episode of NUMB3RS, Amita is asking Charlie a question about exactly where he learned code phrases (beyond that it was at the NSA) and he replies with the trope. She thinks he's kidding, but he's actually quite serious.
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."
Paranoia: one suggested variant on "That information is outside your security clearance" is...well, this trope.
In the second Gabriel KnightAdventure 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.
In The Elder Scrolls V: Skyrim, when the player character joins the Dark Brotherhood and gains access to their sanctuary, many members are often conversing on their latest assassination contracts. One particular line of dialogue is Nazir saying, "It's all thanks to a special technique I developed. I call it the "Razor-Winged Butterfly." I'd teach it to you... but then I'd have to kill you."
Combined this with "The Easy Way or the Hard Way" here, when Master Payne gives two Wulfenbach troopers the choice of his being a good magician (who doesn't reveal how a trick is done) or an evil one (who doesn't leave any evidence there was a trick in the first place).
In this strip Der Kestle declines to reveal information about Airman Higgs to Zeetha, as doing so would require Zeetha to be killed.
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.
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?'
In "Big Boogey Adventure", the movie for The Grim Adventures of Billy & Mandy, Billy asks his future self what he is going to have for lunch on Thursday. His future self replies with the trope name.
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".
Averted by the NSA. Because NSA employees are barred from saying anything about the nature of their work (no matter how mundane), it became vogue, at one point, for them to say this line when friends and family asked. The brass had to put a stop to it because it was overly dramatic and only made people more intrigued.