Let them also sue the bullet manufacturer, magazine manufacturer, shoe company, clothing company (no one goes postal naked), car company if he drove there or bus company, lube company if he lubricated his weapon in any way, youtube if he looked up any gun related vids, his parents and grandparents estate, the bank if he financed his home or landlord.
Sounds like AOC type of decision (Holding bank responsible if there's an oil spill)