More gun homicides per capita happen in states with looser gun laws by and large. The good guy with a gun is largely a myth as compared to “i have a gun and I’m going to use it to kill you”.
The existence of active shooter threats and especially their prevalence shows that the best protection - prevention - isn't working. And in the US, the best possible protection (strict gun ownership laws) is prevented based on an ideology that arguably misreads the Constitution on purpose to keep a whole industry alive.
And should it come to an actual active shooter, the response needs to be a properly equipped and trained police force acting against it rather than sitting it out outside the building. That is not the proverbial "good guy with a gun" because that's implied to be a random untrained civilian on the scene.