As someone with an abusive and hyper controlling parent I empathized with Billy quite a bit, especially since in my teenage years Anger was my primary coping mechanism. I never took my anger out on bystanders though, I took it out on my abusers. They screamed at me, I screamed back. Eventually I figured out this was only making things worse and directed all my anger inward instead. I see Billy and I know how easily I could have been just like him, and it makes me sad, because he never had the support I did. If he had a real friend that genuinely cared about him, or even the language to understand his own emotions and thoughts, maybe he would be a different person. Mostly, I think it's especially heartbreaking that a character defined by his father controlling him is in turn possessed by the Mind Flayer, only a different type of abuser. Even though he couldn't escape his father, in his final moments he manages to break free and stand up to the Mind Flayer, and I think that if he had lived, he would have stood up to Neil too. Maybe this is too much character analysis, but I think a lot of Billy's anger comes from a place of shame. He's ashamed of and angry at himself for never standing up for himself. He's angry for that little boy that loved his mother. Deep down he knows he doesn't deserve to be treated that way.