Having never been trained (trust me, I'm seriously considering it now), my thought process would be: if I could not get out (including those with me) I would try to get myself (and others) into an area and take cover as soon as possible and take up a defensive position. Next would be to get on the cell phone and give 911 our location and description...including letting them know I was armed. All of this seems similar to what one would do should someone break into your house and escape were not possible.
I am well aware that being a civilian and seen in the open with a firearm is a recipe for disaster in such a situation (how would responders know I'm not involved?) but I would obviously want to protect myself and others should we have no other choice.
My question is: what else should (and shouldn't) be done if (god forbid) faced with such a situation?
shoot and fire if necessary. chances are if you are unlucky enough to be in the situation you still will probably be lucky enough to exit the situation(retreat). be vigilant...it is always possible there are other assailants, so if you hear him down the next corridor don't assume you are in clear. also don't assume a civilian won't shoot you if you have a gun and are running or doing anything in the mall(even though you are friendly). don't worry about the cellphone; you will know if you need to draw and fight or of you can retreat.
don't try and be a leader with a bunch of strangers unless the situation obviously presents itself. people are going to panic, act in their own way, scatter, not listen to you, etc. worry about yourself and your family unless you can save a life.
this is all my opinion...you or someone else could agree or disagree
as a child I did picture these scenarios with my toygun that looked real. I am not joking...one too many dirty harry movies. I haven't been following this case, and I haven't pictured this scenario since I was little. You have a valid concern though. malls are a compact area with many exits and a madman can choose this for bodycount and escape routes. the loser might have nowhere else to go too but either way it is a sad story from Oregon...