Three may keep a secret, if two of them are dead.
Selina Kyle: OK, go ahead. Intimidate me. Bully me, if it'll make you feel big. I mean, it's not like you can just kill me.
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]
All this, because your wife stuck her nose into things that were none of her business.
—Nicole Horne, from Max Payne.
Narrator: They knew too much.
Keanu Reeves: We know too much!
Narrator: They went too far.
Keanu Reeves: We went too far!
Narrator: ...Those are my lines.
Keanu Reeves: Those are his lines!
—Pablo Francisco, from his stand-up comedy
You really are a smart man Lieutenant Colonel. Did you ever think that would be the cause of your death?
—Envy from Fullmetal Alchemist: Brotherhood
"He knew Johnny's name and
He spoke it out loud
Now Charlie the barman
Has got him a shroud."
—Narrator in the Doctor Who story "The Gunfighters"
"Four may keep a secret if three of them are dead."
— Benjamin Franklin, summing up this trope quite nicely.
I'm sorry I can't raise you, my friend, but the exact details here need to remain between me and our god. I just thank the Dark One that I didn't need to execute you myself.
"The more that you know, the shorter your life will be."
— Electra, Cowboy Bebop: Knocking On Heaven's Door