It is only an armed population that prevents tyranny. Whether or not Yamamoto actually said that he would not invade mainland America because "there would be a gun behind every blade of grass", I am certain that enemies of the US have thought twice about their invasion plans. How many "mentally unstable" people tried to commit mass murder in police stations? How many mass killings have happened on gun ranges? Would you rather commit your mass murder in a "Gun-Free Zone" or in an "Open Carry Zone"?
The possession of guns (by sane individuals) decreases violent crimes.