From The Maltese Falcon
If you kill me, how are you going get the bird? And if I know you can't afford to kill me, how are you going to scare me into giving it to you?
Well, sir, there are other means of persuasion besides killing and threatening to kill.
Yes, that's... That's true. But, there're none of them any good unless the threat of death is behind them. You see what I mean? If you start something, I'll make it a matter of your having to kill me or call it off.
That's an attitude, sir, that calls for the most delicate judgment on both sides. Because, as you know, sir, in the heat of action men are likely to forget where their best interests lie and let their emotions carry them away.