Spared as a Messenger

You have Seen It a Million Times. Someone (typically an Anti-Hero) demonstrates ability to massacre enemies without batting an eye. And then leaves one alive - so that he could relay a message to the boss. Whether said boss is inclined to Shoot the Messenger is another question.

Playing with a Trope happens too, from the real reason being that the sender is less willing to kill than he shows, to the messenger not making it anyway.

"a redcoat: b-but... you can't send a message if there are no survivors! Paul Revere: That's where you're wrong, pal. No survivors is the message."
 * The Non-Adventures of Wonderella got Wonderella's more Hollywood-ish version of historical events here: