Mass killers are very often on or in withdrawal from psychiatric drugs including antidepressants.
Therefore:
1) Antidepressants shall be added to the list of controlled substances that disqualify users from possessing 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 possessing firearms shall be required to secure arms they may own.
Persons on mind altering drugs or those not properly detoxed of them are not deemed to be fit for service in the militia referenced by the Second Amendment.