I think the single most effective thing we can do to prevent these mass murders is to teach everyone -- everyone that violence in self defense is both a legal right and a moral obligation. Even if you are unarmed, cowering helplessly before one man with one handgun will get you and a lot of other people killed. The dead and wounded in this incident totaled 16 people, deliberately not counting the attacker. Reports are the murderer asked victims whether or not they were Christian, and shot those who said yes in the head, those who said no or refused to answer in the leg.
Had every single one of those people looked their personal incarnation of death in the eye and "Not today!", instead of cowering in terror, answering a religious quiz, and waiting to die, more of them might have lived. No matter how many guns you have, a single person is going to have a hard time shooting 16 people who are actively fighting back.