Someone would have to clearly define the conditions that would preclude an individual from owning a firearm, meaning basically that they’d have to diagnose people with “potentially homicidal wacko” stamp. This shooter supposedly had Aspengers, which in US is considered a social disorder, while some other countries link it to Schizophrenia. Schizophrenics are often violent, which is also a whole another can of worms. Furthermore if someone was diagnosed and precluded from owning a gun, the rational thing to do would be to institutionalize them so they don’t hurt others by other means. That’s an even bigger can of worms. Anti-gun advocates would never agree to institutionalize and/or forcibly medicate people.