I call BS on this. In such a situation there's a lot of confusion and panic about what is exactly happening. It's not like everyone knows how many shooters there are and how they look like. Someone pulling out a gun can easily mistaken to be an attacher and could be attacked himself by someone else trying to be good samaritan while himself being mistook for an attacker by the original good samaritan resulting in chaos. This is not like the movies where everyone knows who the bad and good guys are.