Just illustrating a point. If someone is willing to take another person's life in an offensive fashion, they usually aren't worried about the "good guy with a gun" that might stop them. And if they're really crazy enough, then a "good guy with a gun" isn't going to have a chance in hell to stop them.
Take for example the guy that shot up Vegas a couple years ago from the window of a high rise. What good was a "good guy with a gun" in that situation? How many folks do you think were probably carrying at that country concert? I'm willing to bet at least a hundred. It did nothing. It did nothing to Oswald. A good guy with a gun didn't save Kennedy, or Kennedy, or Reagan, or Lennon, or Tupac, or Selena, or Marvin Gaye, or Gabby Giffords, or Dr. King, or Malcolm X.
Not the kids at Columbine, or SandyHook, or those down in Miami. Not the ones on the campus at UT, or in a Walmart in El Paso, or in the nightclub in Miami. Good guys with a gun have certainly failed to save hundreds in LA, Chicago, and NY and in a hundred other cities around the country.
The only deterrent to a bad guy with a gun is him believing that his life is worth living and that other peoples' are too. A good guy with a gun is like putting a bandaid on a chainsaw wound then telling the victim that you're doing your best to save them. Notice that two people died for no reason before a well prepared church in Texas could even react.