People look for solutions , but really there isn't any in this situation. You have a country that is already saturated with guns and a weird ( from a foreigner's perspective ) sense of entitlement and glorification of guns among a significant majority. Link it with the proliferation of "shooter" computer games ( which desensitises the action) and it is a truly lethal combination.
These kind of incidents are simply going to repeat constantly as there are always madmen / criminals around and the easily availability of guns means they can kill a lot of people in a short time and with the police being armed , they often guarantee their own death in a macabre murder / suicide death wish. Sadly it is something Americans just have to accept , and , in fact , incidents like this just increase the chance of copycat events.