Mikey, mikey... we have, of course, trod this path many times here to for... but, had there been even one legal gun holder at the terrible events you reference, the outcome would have most likely been entirely different.
All[i persons in all of the events in your reference, gained the use of the firearms by illegal methods... one even killing his mother before taking the guns, which he'd had been previously denied the right to own due to his mental illness.
Your proposal would result in [i]only] "bad guys" having firearms. I've said it before, but it bears repeating that the average response time after calling law enforcement personnel is about 15 to 18 minutes... a lifetime in a desperate situation. I'll not delegate the safety of my family to anyone, least of all respected lawmen and women whose only failure is being 15 minutes too late...