The Traitor

Remember that, he told himself. The first friendly thing he does, he will have made a decision.  No, he said to himself, don't fool yourself. You do not know how he was before; but you do know that he is going bad fast and without hiding it. When he starts to hide it he will have made a decision. Remember that, he told himself. The first friendly thing he does, he will have made a decision.

For Whom the Bell Tolls (1940)

Ernest Hemingway

Listen, whoever comes to you with this Barzini meeting, he's the traitor. Don't forget that. So, Barzini will move against you first. He'll set up a meeting with someone that you absolutely trust, guaranteeing your safety. And at that meeting, you'll be assassinated. 

The Godfather (1972)

written by Mario Puzo, Robert Towne, and Francis Ford Coppola

 If you're part of a crew, nobody ever tells you that they're going to kill you, doesn't happen that way. There weren't any arguments or curses like in the movies. See, your murderers come with smiles, they come as your friends, the people who've cared for you all of your life. And they always seem to come at a time that you're at your weakest and most in need of their help. 

Goodfellas (1990)

written by Nicholas Pillegi and Martin Scorsese