No, educate them to realize that a gun does not imply death. The only people who should die in such a situation are before anybody realizes that he has a gun - and obviously they aren't the ones I expect to react. Once it is recognized that the hostile has a gun, the worst possible thing (as measured by kill count) is for everybody to give him control of the situation. Without their cooperation, it is impossible for him to get a clear shot on anyone. A common misconception about people with mental illness is that their brains are incapable of reason. This is blatantly untrue (otherwise they would not be able to dress themselves, much less operate a gun); rather they simple have an extremely skewed sense of values. So the shooter is certainly rational, we don't have to assume.What you are asking is literally every US citizen to do is learn not to be afraid of death?
The second point is taken from a rational point of view, which we cannot assume the shooter in a mass shooting is working from.