Watchtheworld - that's an interesting question (about the lack of empathy in, presumably, non-sn individuals). I don't think you're alone in asking it.
I found one article that was written by a clinical psychologist trying to explain it in simple terms (though he didn't really go into it). But his main thrust was to say the crime was possible because of a lack of empathy but he didn't really go into cause of that. Though he did suggest poor parenting to have a role. (Mind you, his example was Jeffrey Dahmer, and it really wasn't clear that he wasn't non-empathic from early on.)
I guess it's a difficult question. Why? Is it hard-wired in some individuals? Learned in others? Can it be unlearned? Are there different types of non-empathy? (I'm sure there must be).
I had a hunt for the article but couldn't find it, sorry.
Actually, the aspect of this that continues to really surprise me is how full of holes the care system is. and not just in Doncaster. The children who allegedly carried out the attack were in the system, though clearly too late, and in the weeks leading up to the attack were being reported again and again. It speaks of a sort of acceptance of all sorts of behaviour, from poor parenting to extremely dysfunctional behaviour in children, that there is no mechanism for dealing with.
It's awful, really. And the situation is not going to improve because it would be far too expensive. If it's like this after the years of economic plenty, goodness knows what's going to happen in the coming economic and political climate.