Grandpa didn't stop a massacre. He stopped a couple of punk kids that were not interested in shooting. It looks like he spared them as it didn't appear he hit either one of them. Who knows what would've happened in that scenario if the two robbers were more brutal.
Armed people on occasion stop a crime. Clearly. Does that happen more than armed, largely law-abiding citizens getting involved in shootings because they have a gun nearby at the time they decide to get drunk, belligerent, fed up, etc.?
Maybe he would have stopped the theater killings or Killeen or Vag Tech had he been in those places. Who can say.
If the NRA has its way you won't be going anywhere that doesn't involve guns. More is the answer, not less. That seems sad to me, if not wholly misguided. I guess if saturation is the goal, starting and stopping massacres spring from the same set of protocols, liberties, etc. Not a moral equivalent, obviously, but functional access to the means is the same. Great.