The USA is a big place. There is some sense in people who live in remote rural locations owning guns in order to protect themselves from wildlife and law breakers. They could be living many miles from any police organisations.

I suppose a cases could be made that if everyone carried a gun there would be less fatalities from mass shooters. The mass killer would be stopped earlier in his spree. That would probably be offset by an increase in killings as a result of arguements over politics, sport, parking spaces etc., resulting in guns being involved. Unlikely to result in fewer fatalities overall.