That whole idea is poorly thought out. In the split second of danger presented in the situation where a good guy with a gun could stop something, no one has any idea what is happening or why or to who and the decision must be made in seconds. No wonder good guy with gun doesn’t just start blasting. If their goal is to not hurt good people, blasting away is a good way to hurt Innocents.

Not to mention, people don’t like getting shot themselves.