The idea that a good guy with a gun can stop a bad guy with a gun is a pervasive American cultural fantasy.
Allowing private citizens to carry handguns has been found to increase violent crime by 13 to 15 per cent.
The world does not exist in the binary of 'good guys' and 'bad guys'. Even if ensuring everyone had guns did mean 'good guys' could stop 'bad guys' from committing crimes, this does not address all the gun deaths and injuries that are not the result of criminal intent, such as suicide and accidents.
The fantasy that this would work, also, is a complete fallacy. Any number of things can go wrong with using a gun to deter a shooter, even if the intent is good. A bystander may be shot, or a criminal may just steal a 'good guy's' gun. The more people overall that own a gun, the higher the homicide rate.
Those who do own a gun are more likely to be killed or injured than those who don't.