From WoWWiki

Kill stealing is rushing to kill a mob some other player was heading towards, just to prevent them from getting credit for killing that creature or to get an item drop before they do. This is especially bad when the mob is somewhat unique or difficult to get to.

The Defias Messenger of Westfall is an especially notable one, because he is unaggressive until he is attacked, and he roams a wide area so that he's difficult to find even if someone hasn't killed him.