The response of many gun rights people is to say, "If the theater didn't have a no-guns policy, someone could have returned fire." True. And there have been cases where this has worked out, such as Jeanne Assam's shooting of a murderer in the lobby of the church in Colorado Springs several years. But The Joker was wearing body armor, and throwing tear gas. Returning fire under those circumstances had more chance of success than praying for a meteor strike to take out the bad guy, but the core problem here is that someone clearly was mentally ill--and we no longer make any serious efforts as a society to help mentally ill people.
UPDATE: This account describes the "urban assault vest" that The Joker was wearing as costing $300. That's too cheap to be bulletproof. This report points to the vest, which does not appear bulletproof. Still, he was wearing black (making him hard to see), wearing a Kevlar helmet. This would have been a very difficult shot to make in darkness, with tear gas.
UPDATE 2: A reader points to a picture in this article that could be soft body armor. One problem is that the tactical nylon vest isn't necessarily much different in dimensions from soft body armor.
UPDATE 3: It does appear that the vest was not bulletproof. This would still have been a hard shot to make in the darkness, but consider the alternative.