My limited understanding of the US standpoint starts like this:-
1. Many live more than 20 minutes', at police-car speeds, from law enforcement assistance.
2. If you live on a farm, your nearest neighbour is too far away to hear shouting, screams, violence or even gunfire. If trouble comes to your doorstep, you'll need deadly force.
3. If you live close to forested areas, you will eventually find yourself in the 'bear in the backyard' situation (also wolves, moutain lion etc).
Other reasons range from entertainment to paranoid fears of totalitarian government, bordering on sedition but I'd like to avoid expanding on those.
These are views from 'normal citizen' types that I've seen expressed on Facebook threads, which repeat, ad nauseam, every time there's an incident on this scale.
If you can suggest fixes for the above problems - of the scale of their geography, their most dangerous wildlife and the criminal/homicidal element of their society, without resorting to automatic weapons, then you might have solved the problem of mass shootings.
As pointed out above there are still other, worse, ways to be done in.