Two things in your post. You're taking examples of the extremes and disregarding a large group of other gun owners (hunters). There are nearly 14 million hunters that go out into the woods each year. The average number of deaths (usually accidental) is around 75. That's a very small percentage. And they weren't accidental. Total number of gun owners is somewhere around 50 million. I don't agree with the stats used to compile, but mass shootings are somewhere around 200 per year. Again, very small averages. To hear people say they are afraid to go the mall or a movie theater is ridiculous. They have a better chance of dying in a car accident or getting hit by lighting.
A good guy with a gun is what stops a bad guy with a gun. Usually, it's the police. I don't think people should carry a gun so that they can jump in and stop a mass shooter. I don't conceal carry myself, mainly because I think the potential need is minuscule. Trying to stop a mass shooter on your own would open up too many other liabilities. Getting shot yourself by LE or shooting an innocent person by mistake would not make it worth it. The first response to an active shooter should be to flee. If you can't flee, then shelter in place. If you are found, then fight. At the point that you have to fight, that is where a gun would be effective.