It sounds like he *is* mad. A lot of times when someone is harmed at a young age, they have a lot of rage that takes them a lot of time, support and hard work to solve. My cousin was adopted from an orphanage in another country where he was not attended to emotionally at all. He was in a crib and nobody would come if he cried, the back of his hair was worn off from simply lying there, he would get fed and his diapers changed and that was it. He came away very damaged, and they adopted him no older than 18 months old. His question to his mother later on (when he could articulate it) was, "Why am I so mad all the time?" But of course, he had the right to be. You are quick to label the child abusive (and it is probably one way to say it) but don't talk about what he must have gone through to get that way. Kids aren't born that way, you know.
Is there any way you can get to a family counselor or someone in the school system for advice? Naturally, you have to protect the animals and your other child, but this child, you can't simply point the finger and call him bad, you have to help him too. If there aren't any resources at all, please talk to a public health agency. You need help and especially he needs help.