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.

As a Death Trope, all Spoilers will be unmarked ahead. Beware.

Examples of Spared as a Messenger include:

Film

• 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:

Mallory: When you tell people what went on here, tell 'em Mickey and Mallory Knox did this. Understand?
Pinball Cowboy: (nods)

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.

Video Games

• In Total War Shogun 2, in one of the motivational pre-battle speech cutscenes there is the line:

When we are done, I want the Tokugawa to talk of this day for a hundred years! We must leave one alive to tell of our deeds!

Web Comics

• 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:

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.

• In Oglaf Kronar invokes this trope. The designated messenger then proceeds to tell everyone that Kronar has gone soft.
• Errant Story had monks of the Ensigerum Order wiping out groups of Gewehr Wraith (rival assassin organization encroaching on their territory) in the Ninjas vs. Cowboys war. Their SOP is to "leave one alive to make sure the Gewehr knows we did this"... "As if we wouldn't have been able to guess".