I think the best thing to speculate is what would the perp have done differently if there were anything like 10% or 20% of U.S. citizens carrying concealed handguns?
Good point. In some cases though, in some mental cases i should say, it may not have made a difference. However, it's true that most murders/criminal types prey upon easy victims and therefore avoid places where cops are nearby. So if a number of people are armed like cops, and trained how to react in situations like this, it would give many would-be murderers second thoughts.
Here though, if the guy did in fact use tear gas or something of that sort, then he might have suspected the possibility of people being armed (unless the theater had metal detectors at the entrance) and took precautions beyond just wearing body armor.