I want to make sure I understand your argument here. So you've said that if people are determined to commit a mass shooting, things like laws or restricted access to guns will not stop them. However, having to face armed resistance would? Why? If they are so determined, why would that stop them? Wouldn't that just be another obstacle they would have to overcome?
No, because mass shooters WANT to kill or hurt their victims. Armed victims would pose a challenge because they might shoot back. It's not a secret that criminals are less likely to rob houses if they know that the occupants have a gun.