If you or I had the motivation and the mentality, we could kill any number of children. Whether or not we had a semi automatic rifle is not really relevant.
What's the difference between Harold Shipman and this bloke, or Anders Breivik? None, really.
It is nothing to do with paranoia, or with arms.
It is slightly to do with help available to the mentally ill, as you suggest, but that is part of a much broader scheme.