Made illegal : Gun free zones. These were a bad idea from the beginning, as they prevent bad guys from being stopped by citizens. Gun free zones give the deranged free reign to kill many more before armed law enforcement can arrive.
Made legal : Much tighter gun control. Anyone having a previous history of mental instability or violent behavior, will be permanently banned from purchasing a firearm. Data to support this law may be pulled from law enforcement, medical and workforce databases. Everyone must be required to take weapons safety, anger management and basic marksmanship courses before being allowed to purchase a firearm.