Spared as a Messenger

You have Seen It a Million Times. Someone (typically an Anti-Hero, but happens to variety of types) 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.

Films
"Mallory: When you tell people what went on here, tell 'em Mickey and Mallory Knox did this. Understand? Pinball Cowboy: (nods)"
 * Doubly Subverted in Mulan. The Huns (lead by Shan-Yu) capture two scouts. Shan-Yu lets them both go, to tell the Imperial Army that the Huns are coming. Once they're out of earshot, Shan-Yu asks "How many men does it take to deliver a message?" The archer standing beside him draws his bow and aims ...
 * Natural Born Killers:

Literature

 * In The Dark Elf Trilogy after a brief border skirmish deep gnomes Drizzt argued to let the surviving gnome go and tell his kin they got no chance there.

Webcomics
"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."
 * Schlock Mercenary, during their vendetta with Pranger's team: Theo spells out how this works.
 * The Non-Adventures of Wonderella got Wonderella's more Hollywood-ish version of historical events here: