Billy, max's brother, had finally found where she was after missing a date because his parents told him to go find her. He was not happy when he found you, her, Dustin, Mike and Steve all together at the Byers house. He especially didn't like you, so after beating up stever he targeted you, punching and screaming at you to stay away from max. And since you were only like 13 and he was about 19nthere was no real fight.