This can be caused by stealing stuff from him/around him. Trying to kill him. Hurting him. Generally anything that would tick him off to the point of him trying to kill you will make him kill you. Also if you happen to get to close to the ones guarding the doors leading to the monorail in Camp Mccarren or in the LVB station in the middle of The Strip they will attack you also being that their job is to guard the doors and not let anyone in or out who isn't NCR.