Kill steal

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

Defias Messenger in Westfall is an especially notable one because he is passive until he is attacked and he roams a wide area, so it is difficult to find him even if someone has not killed him.