Alice and Bob get into a competitive one-up competition about who would have killed the other one first if they'd wanted the other one dead.
Alice: "If you wanted me dead, you'd have killed me already." Bob: "I knew I was forgetting something." BANG!
Zig Zagged: Alice takes Bob at his word, but we find out this gives him the ability to kill her. Then he still decides not to. Except this was another ploy to save face. Except he has, by this point, Become The Mask, and now really doesn't want to kill her. But that doesn't mean he can't hurt her...which he doesn't anyway.
Bob has the ability to kill Alice, but doesn't and doesn't ever ask for her trust.
Alternatively, Bob has the ability to kill Alice, and he does.
Bob lacks the ability to kill Alice, and everyone knows it, so this doesn't come up.
Enforced: "We need to make it believable that Alice would get in the mob boss' car. Make him point out that he wouldn't need to lure her there to kill her."
Lampshaded: "Is this the part where you try to get me to trust you by saying if you wanted me dead, I'd be dead?"
Invoked: "Tell Alice to get over here. She's knows she'd already be dead if I wanted to kill her."
Exploited: Bob deliberate refrains from killing Alice when the opportunity arises, so he can use this trope to gain her trust. He can kill her just as easily at a later date.
Defied: ...and after Alice arrives, Bob shoots her and says, "What? I wanted her dead!"
Discussed: "I think he's gonna say something like how we know he won't kill us because we'd be dead already." "You would be." "Well, yeah."
Conversed: "Why does she think he could kill her at the drop of a hat? Stuff like that takes planning."
Deconstructed: This trope is used to save face, the assassination attempt really was botched.
Reconstructed: However, the fact that Alice managed to avoid it gives Bob enough respect for her that he decides not to kill her.
Played For Laughs: Bob is a harmless person, and the people laugh at this and trust him because he can't hurt them, not because he won't.