If you believe that the shooter in this case was disturbed or had mental illness (I've seen no reports to conclude this, btw) then the first thing you would demand is 1) better mental health care facilities and treatments for the public at large and 2) universal health care systems to provide the least possible friction in accessing those mental health care services.
Also, you don't have to be either for banning guns or having no regulations at all. Instead, a compromise of allowing regulated gun ownership would be better.
Also, you don't have to be either for banning guns or having no regulations at all. Instead, a compromise of allowing regulated gun ownership would be better.