He's absolutely right when he says the blame cannot be laid at the feet of the National Rifle Association, or the guns the killers used. The guns were just tools used by the killers.
He's wrong when he lays the blame on the perceived lack of God in the schools. As an atheist, I cannot agree that those who do not believe in a supreme being must necessarily "allow evil, prejudice, and hatred to rush in and wreak havoc". It is possible to be a good person without being intimidated into doing so by fear of divine retribution. Indeed, that is the goal to which I strive (imperfectly, since I am human).
The problem at Columbine High School was simply that two guys who had put up with far too much from their peers decided they weren't going to take it any more. I've been there. I don't know why I didn't grab a couple of guns and shoot up my high school, but I have no trouble at all understanding why others have done so.