Did you know that Cruz and Lanza were on psychoactive drugs according to family members and acquaintances? Holmes was on a psychoactive medication.
Some people shouldn’t possess firearms, at least for defined periods of treatment or drug addiction. Others shouldn’t possess firearms at all (obvious mental retardation, etc.).
Yes, the Founding Fathers were talking about sane people. Still, I worry that a Mrs. Bill Clinton might declare any and all conservatives as insane. One congresscritter already said that veterans should be prevented from buying guns because all veterans are "obviously" insane.