We have all put together in our head the way we think the world works. If x goes into process A, then y will come out of it. But if we all have a different understanding of how the black box of mystery that is Process A actually works, we will come to very different conclusions. So if one person's life experiences and the explanation for the world that they received growing up said that guns are powerful but necessary tools that help keep safe the people we love, then it's natural to say that people at a mass shooting would be safer if a few responsible people with guns were there.
But if you were raised only seeing guns in the movies, only as tools of violence and destruction, if you've never seen a need for one in your own direct experience, and no one has explained to you the legitimate uses for guns and how they can be a natural, even reassuring and comforting thing, then the idea that a chaotic scene of carnage would be limited or even stopped by private citizens with guns would seem ludicrous. You're likely instead to think that more guns would have only led to more chaos, more madness and death.
We all have ideas in our heads about how the world really works.