I voted shooter. I thought about mental health system, but I really don't know enough about this shooter's history to make a determination that it was the mental health system's fault. Although the mother of the shooter may or may not have acted irresponsibly (not sure on this point either), I don't think I can actually blame her for the actions of her obviously mentally ill son.
I would also vote gun-free zones because I believe that all the people contained within a gun-free zone are sitting ducks to lunatics like this and are left completely unable to defend themselves. If just ONE person had a gun, they could have put an end to this massacre. That is a real tragedy.