arma virumque cano (et alia)
We know nothing about who the shooter is or what the motivation is. Short of a total ban on privately owned firearms and continual sweeps to round up any that are privately held, what would have prevented this? Don't guess, and don't feel. Give a specific answer.
It's not as all-or-nothing as you keep pretending. Proper gun control laws, as I've outlined, would weed out many undesirables as well as cut down on the gun flow to criminals.But, you don't want that it might inconvenience you.
And I've pointed out to you that your proposals are exactly what exist in Washington, D.C. and in Chicago. Citizens who try to follow the rules have to go through a thicket of red tape, pay ridiculous fees, and buy only from approved lists. They can't carry their guns in those areas legally. That's the control that you want. That's a denial of rights, not an inconvenience.
It's not that I think there's nothing we can do, it's that I don't care.