Not that I feel any desire to get into this debate, but do want to provide some thought, on at least 1 wee aspect.
Armed cops are ambushed and killed all the time. Why might an armed civilian be any better at preventing, or avoiding it. It's logically inconsistent to think Johnny Q would be any more capable (likely less given most/many cops have something called street sense and situational awareness, likely not found in the average person) at not becoming statistics.