As Ezra notes, one of the few things that actually could have prevented last weekend's tragic events is if Loughner had receives proper mental health care. Apparently, Loughner's logic professor thought he had a messed up brain, although didn't foresee any violence. The narrative here thus becomes, How is our mental health system failing?
But, of course, it's not nearly that simple. Even if Pima Community College had the world's finest mental health care system, there's no guarantee Loughner would have availed himself of it. Several people reportedly advised him to seek help; he chose to drop out instead.
We can call this "slipping through the cracks," but what's the alternative? College professors who can force a student to undergo psychiatric care? Are we even remotely qualified to exercise that kind of power? How many quirky but otherwise harmless students would get caught up in that net?