Mass killers are very commonly on or in withdrawal from psychiatric drugs, including antidepressants.
Therefore:
1) Antidepressants shall be added to the list of controlled substances disqualifying users from possession of firearms.
2) There shall be a national registry of psychiatric drug users prohibited from firearms ownership.
3) Persons living in homes with persons prohibited from owning firearms shall be required to secure such arms.
Persons on mind altering drugs are not deemed to be suitable for service in the militia referenced by the Second Amendment any more than persons in prison or mental institutions.