The Serial Killer is seen killing someone with a shotgun on a dock during the mission Missed the Boat?. He then quickly runs to a car and leaves. While driving away, he tends to kill another pedestrian in a hit and run attack. If the player pursues him, he will attack the player. It is most likely that he was placed there during the mission, as a dock would be a nice place for a little easter egg, as the player would likely be more focused on the mission if they were in that area.