It is hard to say that even if a person had a concealed weapon, that this would not have gone down any differently. Like at the 2011 shooting of Gabby Giffords and 18 other people, there plenty of people who had gun on them with a legal a conceal and carry permit. Yet Jared Lee Loughner was tackled by bystanders when he was reloading, he wasn't shot. One of the people who tackled Loughner had a gun, but he made the split second calculation not to use his weapon. When there is chaos and people running everywhere and tear gas that you are choking on, it can be hard to discern who to shoot at. Even if you knew who to shoot at, you could easily hit a person trying to escape the crowded theater. Not to mention that this guy was decked out a ballistic helmet, ballistic leggings, groin protecter, tactical gloves and gasmask.
So it isn't as simple as "if somebody had a gun on them, this would have ended differently". I honestly doubt it would have.