This actually happens with some regularity, it's just that prevented mass shootings dont get the same attention. (I realize that in this case the shooting may have been over, but the general point still stands).

So when a mass shooting happens where the assailant is sadly not stopped and kills lots of people, you get the "hurr durr wheres the good guy with a gun?" bs because people are not aware of incidents like this.

Good guys with guns defend themselves and stop crimes thousands of times a year. They just dont get a month of news coverage and a Bloomberg financed march.

CDC study that they decided not to release in the 90s actually confirmed the higher end estimates of over 2 million defensive gun uses per year.

https://www.google.com/amp/s/reason.com/blog/2018/04/20/cdc-provides-more-evidence-that-plenty-o/amp

Remember that this includes everything from killing an assailant, down to simply brandishing a gun to dissuade a potential attacker.

The lowest end estimate, from an antigun group, is 70,000 a year. Which is still 7x our gun homicides.