The vast majority of Americans are not gun users for the most part and even less have the ability to quickly determine who is the bad guy. What is scarier than 1 guy with a gun? I would say a store full of people packing who have no idea what they are doing when 1 guy comes in with a gun. Picture it now, gun comes in with a pistol and starts shooting. Gun owner 1 pulls gun, gun owner 2-50 pull their guns, now how do they determine who was the first person with the gun? You now have 51 people shooting at 50 people with guns with bystanders everywhere. If someone is fully trained with guns, their used, how to shoot tactically and identify the source then that is one thing but the blanket "good guy with a gun" gets dangerous very fast.