WAAAAAAY to many variables to answer this question honestly, and in a way that makes sense.
lets put it this way, this dirt bag knew he was choosing a movie theater that was a "gun free safe zone". he knew the chances of return fire were slim to non. thats certainly a possibility why he chose to basically 'corner' his victims.
back to the OP'S QUESTION: "Would an armed person have changed the results in Aurora?"
as much as id love to be like "ya, i told you so, the dirt-bag wouldnt have killed as many, see - we conservatives are right, blaa blaa blaa"... i simply cant say that.
It all depends on so many variables. and iv tossed this around in my head a few times already. you have so many elements of the movie theater that makes for a really tense shooting environment: 1)dark 2)noisy) 3)lots of people) 4)elevation 5)flickering light 6)distance 7)element of surprise 8)thin isles 9)stairs = for the major ones. The movie theater is a death trap - if you really look at it from a tactical stand-point of view. Plus he was almost flush with the screen, so he is in the darkest spot of the movie theater if you think about it.
i drew a picture depicting what i think it looked liked. the green triangles are really the only spots that could have had a good shot. very few, and limited - and frankly...
I DONT THINK MUCH WOULD HAVE BEEN CHANGED HAD THERE BEEN PEOPLE CARRYING WEAPONS THERE. The first thing anyone who carries would have done is dropped to the floor and shielded their loved ones. then possibly thought about taking a shot. but with someone shooting a round off, almost every second - it wouldnt have given anyone time to even risk sticking your head up.
To many things going against the victims. too many variables. to many people. to short of a time. he didnt move around much. he was in the darkest spot of the movie theater.
did he shoot the first few rows first? if not - that would have given the front row time to return fire.
did he come in with guns pointed to the air like a rambo type, to announce his presence?
did he only shoot at the stairs? did he shoot at the isles?