So the likelihood of pulling out your weapon, yelling, "Die scumbag!", shooting the guy dead with no muss and no fuss would be highly unlikely without hurting or killing someone else and maybe losing your own life.
Why in the hell would you yell anything just shoot the freak. if he has a vest on you shoot him in the face, or take his legs out. its going to be hard to shoot with 10 shots in your legs,nuts,hips.
It's not so easy to aim under those conditions much less think clearly. And you have no clue what you would do in that situation unless you have had siginficant training. Any training will teach you to avoid a shootout if possible. The point is not to use your weapon, but only when there's no other option. Without any training, chances are you will panic along with the rest. Fear is overwhelming when your faced with those situations. If not, chances are you will do something stupid to compound the issue. While you're thinking about killing the bastard, you're also putting other people at risk. Not from him, from you. Now you're just as much a threat as the actual shooter. And by the way, the cops won't care too much if you were trying to do the right thing. You're just another guy shooting in the theatre.
I hope you never face a situation like that. The reality is not like the movies.