I wouldn't worry about it either. The guy you beat up was obviously talking out of his ass when he said he was going to get you on simple assault charges. Like I said, assault is threatening someone, it involves no physical contact and if he showed up at your girls place looking for a fight, that is assault. You could get him on that. Only thing I would worry about is if you beat him too badly, he could get you on battery charges whether you were defending yourself or not. If it was just a couple of punches here and there, you are all set. You can use self defense. And don't forget, it was him who was looking for the fight, not you. If he keeps following you around and harrassing you though, I wouldn't hesitate to file assault charges on him. Good luck bro.