<human>: Could you distill the essence of the text into 1-3 sentences?

My brother is nine years old. Since last year I have noticed some clear changes in his personality and behavior. He was very sweet and liked to buy me presents on occasionally (With mom's money). He would lend me his things and loved me reading bedtime stories.

It started when he was diagnosed with ADHD. He was given medications, the natural kind, and he became violent. I figured it was a change due to the medications but during summer he stopped taking them and nothing changed. He started lying, manipulating my sister and me into getting in trouble and going as far as to cry while acting. 

He started menacing to hit people if they touched his things, he became addicted to DS games and computer games really quick. He wasn't the lovable little brother I knew anymore. Then he started hitting people. Mom thought he might be autistic. She is a teacher and has seen many children with autism. He passed tests and they came out negative.

He then started hitting our cat, throwing it down the stairs and stuff. Being an animal lover, I told him it was wrong and he was hurting the cat. He didn't listen to me and even hit me for it. At a certain point I told mom to do something about him, I mean, it's animal abuse. She told me she knew better then me how to raise a kid and that it was normal to be rough as a kid, for boys. However he is more then rough. I have seen him strangle the cat, hold her up by her tail, etc.

I decided to let it go and forget it all but a few weeks earlier, when my uncle died, my brother made comments about how it was funny that his skin was so stiff and cold. It gave me shivers. Especially since he was close to my uncle. Or at least it looked like it.

I have talked about this with my mother, who told me I was over thinking it and that my brother was HER responsibility. The more it goes, the more she spoils him. She doesn't even realize it. 

I don't know who to go to for help and I don't know if I'm overreacting. Help me out, Reddit?
<bot>: My 9 years old brother abuses cats and hits people, he is manipulative and shows selfish behavior a lot more then your average kid. Could he be a sociopath? If not, can anyone tell me what he thinks is wrong?