This always interested me.
You're right this is Real life.
I was always trained to identify your target before firing.
In this case it's midnight, it's dark, you're unprepared.
Suddenly, smoke hits, gunfire rings out, people start screaming. I identify the shooter on the bottom left. You're behind me, and react quickly and start firing. Now I have two targets. A third person gets in, he's not really trained, he's firing wildly. Now I have three targets.
I don't know who's hostile.
I do know, I'll take out my first target on the bottom left. Now, how do I know you're not a hostile target? I'm willing to bet I have better training than you... you and that joe schmore are next on my list to take out... better safe than sorry, no?
Conversly, how do you know I'm not an aggressor? Will you shoot me too? Will you go after joe schome who's ducking behind the seats, but still shooting his pistol over it and at everything?
3 other armed people firing in the theater, not including you. You don't know any of them. How do you identify your targets?