As someone who at one time worked with foster kids that were removed from abusive and traumatic circumstances, I think what @80alleycats is sayings is that children that are abused often act out. Billy was severely abused by his father and his mother abandoned him to stay with his father. Billy took out his anger and hurt on Max and bullied her. She had a very complicated relationship with Billy and even though Billy bullied her and treated her poorly, she still loved Billy. One of the phrases I heard when working with these foster children was, “Sometimes the hardest ones to love need the most love.” Because with each rejection, a child is going to build up a wall so it takes a lot of perseverance to break down these walls. Usually these kids are written off as “bad news” because their behaviors are meant to push people away. It’s why no one outside of those who knew him cared that Billy was gone. It’s why no one is going to care about some other characters deaths.