I would guess that applied technology knows how to develop artifical lights, that emit a portion of the light spectrum, that disallows regular human vision. Perhaps, a light that is temporarily blinding while it is on. Now, connect this to a device that turns the light on at the first sound of a firearm. With such a device put in where the public is, a mass murderer, with a gun, would only get one shot out, before he would not see another potential target.
If smoke alarms, CO alarms, can be mandated in our society, a device to disallow regular vision could also be manadated in my opinion. Or, perhaps a device that emits a sound wave that causes dizziness.
In other words, applied science might be able to assist society's need to deal with tragedies like we saw.