Our Youngest Son Is 6. He Is My Step Son

Our youngest son is 6. He is my step son but is always showed lots of love. But if there isn't an adult in site he violently hurts kids... bigger smaller boys girls. And lastnight he almost killed my 4 yr old nephew. He had guillian Berra and his trachea is out but has still a small hole, my stepson has told me several times he doesn't like him because he hugs him and tries to play with him all the time. My nephew has leg braces still and was sitting on the couch I walked out of the room and came back in a few mins later and my stepson was choking him badly he couldn't breathe. He hurts gim everytime he is here and I have copies of behavior charts from this past school yr and there are more days that he hurt kids than good. He punches in the face kicks with boots bites until they bleed and just basically attacks kids when they make him mad and most of the time they really haven't done anything to him. He wets the bed still and will hold his pee until he can't anymore and pee in his pants/shorts. He also hurts animals. I am very afraid that he's going to hurt a child bad or possibly kill one after lastnight as of lastnight. Had I not walked back in the room so quickly I honestly belive he would've choked gim to death he wouldn't let go. Plz any advice or ideas as to what is going on. I don't want him to grow up and hurt or kill somebody and go to jail. I want the best for all my babies I just don't know what to do. We have rules and his biological mother let's him cuss. He bullies and when he makes kids cry he laughs at them. I'm at my wits end and don't know what to do. I've Googled some of these symptoms and read a lot about these being signs of very bad things. Please help us