When I hear someone say they need a gun to STOP a mass shooter, what I hear is, “I would rather play Wild West than do something that would actually save lives.” It’s a fantasy that is, like you said, fun. It’s fun to imagine yourself a hero stopping a mass shooter, but no, it’s not a good enough reason to own that kind of weapon, to take that kind of risk, in the off-chance you get to play hero (and actually do a good job of it).