Sadly things like this are pretty much an every day occurrence here in the US for whatever reason people prescribe to. It's definitely a different world and my wife and I look at things and situations very differently than we used to back in the day. We've always enjoyed going to concerts, but even those offer a bit of anxiety these days as you are checking your exits and trying to anticipate anything that might happen.
I hate that we (society) live in a world such as this but it hardly surprises me when I hear things like this. It'll happen again, there, here, somewhere else...it'll not be the last time.
I'm pretty aware when I'm out and about and capable (and willing) to act against those doing this sort of thing; I'm not the run away sort of dude and am pretty comfortable with sending someone to hell. Cops hate that sort of talk, *leave it to the professionals and all, but they're mostly inept and too afraid to act (here at least) for various reasons so it's left to those who want to stand up to the fuckassery.
I like that you're aware, knowing the escape routes, where cover and concealment is, is shit that could save lives.
Yeah it's pretty sad. I honestly don't know how I would handle myself if I were ever in a situation like that. With zero training I would still like to think my instinct would be to help others as much as I could.
Instinct counts for much and has helped the human race survive. Some run away from danger and some run into it, we need both types of people.