Originally Posted By: ben-t
I might be wrong, but if he had been legally determined to be mentally unstable it would have shown up during the instant background check.

Only if he'd been adjudged mentally incompetent by a court and committed to a mental institution.
And it is MUCH harder to do that today than 30 years ago, before advocates of rights for the mentally handicapped wrecked the mental health institutions of the USA, loosing a flood of "street people" or "homeless" on us. They were, supposedly, to have their needs met by community mental health centers which either were not funded or which they were too crazy to use.
Thirty years ago Cho's deranged, violent writings may have been enough for a competency hearing in Virginia. Not today.