If he isn't at his usual location in front of the tower, someone else may be doing the quest. He will reappear once the other player finishes escorting him.
Because of his low level and Moonbrook's proximity to Stranglethorn Vale, it was common on some realms for Horde players to kill the Defias Traitor as a form of griefing. Alliance players had to look out for rogues while escorting him, especially near the hideout.