I hate to say this, but looking at it from an engineering standpoint, firearms might actually be a limiting factor. I wondered when the first big concert/mass assembly attack was going to happen with a firearm. This evil a$$hole was clever enough to add attacking from a height, where locating him and returning fire were made much more difficult. And he had a lair were firearms and ammunition could be stockpiled. Such a combination of events may not happen again. Even so, he killed only 58 people. I say this with an eye toward macro events, and understanding with humble sorrow how bad it sounds.
Unfortunately, the science of killing people is not in it's infancy. Militaries know that riflemen hold ground, but to really get efficient at killing you need to bring in artillery or air power.
Guns are demonized, but in that morbid list of criminal mass killing events, firearms are behind kinetic energy(aircraft and trucks), arson, and bombings. Banning guns and ignoring underlying causes will make our criminals turn to more efficient methods, while taking the most effective defensive measures away from law-abiding people.