For me, if the “good guy” with the gun is in their own home — GREAT, use a weapon for protection. If the “good guy” with a gun is in the store where I am shopping, restaurant where I am dining, my child’s school…- please leave the gun at home, UNLESS he/she has combat training. I’d prefer to depend on the odds — odds are, the vast majority of public places, at any given moment, WILL not be subject to an armed criminal, and the majority of armed criminals have a goal other than to kill.
I have been (very) present during two armed bank robberies, and participated in highly realistic active shooter drills at another place of employment. Most gun owners, (unless combat military or LEO), don’t receive one-time, much less ongoing gun proficiency & safety training. FAR FEWER have experienced training (or life experience) in high stress situations. Adrenaline/stress reactions can be overwhelming, and I posit, (based on my personal responses and observation of others), few people are likely to do well handling a firearm under that kind of pressure.