Were you looking for the poster in Kargath titled KILL ON SIGHT?

The term kill on sight (usually abbreviated to "KOS" or "KoS") is a status of enemies, usually towards other player characters in PvP, to members of an organization like a guild. Supposedly, when these enemies are seen by members of the organization they are supposed to try to attack and kill the target if possible.

In World of Warcraft Edit

WoW Icon 16x16 This section concerns content exclusive to World of Warcraft.

In World of Warcraft, there is no formal mechanism for tracking these enemies, but some other MMORPGs allow you to make an in-game list and sometimes even highlight these targets in the game in some way.

Opposite faction guard NPCs (Horde 15 guards when detecting Alliance 15 players or Alliance 15 guards when detecting Horde 15 players) are set to KoS if you get within aggro range.