Where does insanity begins?

The only argument of the pro-gun lobby towards reducing gun violence is to prevent crazy folks from having access to fire-arms. Please, show me where is the delineation between healthy minds and insanity?

My neighbor's dog never bites, has never bitten anyone until the day it begins. One doesn't need to be patently insane to act on a whim and use a fire-arm because he or she cannot longer stand a given situation.

It's not the level of sanity that is the problem, it's the proximity and availability of guns!