From what I understand the only prior run in with the law was a speeding ticket! If he were that crazy, being involuntarily committed, scaring family members etc...he would have run afoul of the law before this.
He obviously wasn't. Graduated college with no problems, got accepted into a PhD program with no problems. If he showed up for his interview looking as the potted plant, does anyone think the PhD program would have accepted him? He was also able to legally purchase guns. That again goes against some long established history of mental illness.