Is It My Fault?

About six months ago i moved in with my dad. My brother and i have always been very close and leaving him was one of that hardest things i had to do. Change had never been one of my brothers strong points so i was a little nervous on leaving. Ever since i had left Justin's behavior had changed for the worse. He started getting more and more aggressive, his attitude had become more negative, and his focus had disappeared. I feel as if this is my fault. All growing up i was kinnda like his security blanket. i always had to be there when he went to bed, always was one of the first people he wanted when he was sick, and had been there with him his hole life. About a week after i was gone was when he started changing. A few times a week my mom would get a call from the school saying that Justin had been sent to the principals office for hitting or refusing to do his work, and this had happened in the past but never as frequently. Also, his behavior at home had spiraled out of control. I remember getting phone calls from my mother having her tell me how he had hit the dog, or tackled her and started hitting her with a toy sword. The phone calls also consisted of how he would come home and destroy the house. Screaming that he was going to burn down the house which he obviously wont but hearing that from a 10 year old child is upsetting. She comes to me telling me how he hits the teachers at school. And the thing is, none of this was ever as bad as it is now when i was with him on a regular basis. He never thought of hitting my mother when i was home or he never once screamed that he was going to cut his hands off. A part of me if filled with guilt from seeing what a struggle Justin has become. Most days i wonder, if i did go and move back in with my mother, would anything change? Am i being selfish for leaving him to make my situation better? I understand that i am not the parent but seeing my baby brother living his life in anger and sorrow day by day is a heart breaking experience. i leave myself wondering, does anyone else feel the same? Can anyone help me out of this hole of emotion i have dug myself into? If so, im reaching out to you with open arms and an open heart. I would love to answer any questions that you have. (: