There are foster parents without other children in the home willing to take in kids like this.
And if a parent goes before a family court judge and gives a damn good reason — like this kid’s psycho behavior — for wishing to surrender custody to the state, a judge will go along with it.
And even if such a home isn’t really a “help” to the boy, at least he’s not gonna be able to hurt his siblings.
Because such a calloused rejection by one’s parents wouldn’t scar the most normal of children right? As a parent myself, it’s easy to recognize that no sane parent would give your suggestion even a moments thought.