they said that mass shooters might be mentally and emotionally unstable...
Anyone who derives joy from shooting their fellow beings should be insane and kill the exact way he or she shoots their fellow beings.
Mass shootings are what we learn of happening in the US, not always, but it happen from time to time because their citizens are allowed to use guns.
You don't give a civilian gun when they know that they can have you eliminated in the slightest disagreement or provocation. Many people have been killed because of this. Those who are mentally unstable see guns as a way to eliminate many people for fun or anger.