I can't work out what Voldemort would want with children, but then I don't sit about imagining how I'd carry off evil plans if I were an evil overlord who wanted to- well, what exactly? I've no idea what he could possibly be about if he's behind this.

But I do see what you mean: if you accept the first premise and then imagine that your brother's been made to do these things, from there it's not hard to understand the violence of his self-punishment. He must feel quite desperate.