Yeah because everyone's a real crack shot in a small, crowded, dark concert hall with hundreds of people screaming and panicking. The most unsettling part of this whole "good guy with a gun" schtick after every mass killing is the subtext, "if I had been there with my gun, I would have stopped this".
I get the self defense stuff, I really do. If you're talking about someone defending their home from intruders, stopping assailants on the street, sure. I can buy the argument and I can understand where you're coming from. But it's just an asinine power fantasy to imagine that a French John McClane could have stopped 4 assailants with automatic weapons in a tiny cramped concert hall with all the panic and confusion. This isn't a Die Hard movie.