Yo, Dead...
Some people just don’t get it...and probably never will...
LFOD, brother.
It is stated that the perp was wearing full body armor - head to toe - if you happened to have a Desert Eagle .50 somehow concealed, then you may have been able to get his attention. Anything less would have not stopped him, IMHO. This lunatic set up a kill box, executed his plan, and walked out of the theater.
Perhaps someone could have rushed him during a magazine change...but with the smoke and the bodies on the floor and the aisles crowded with people... A professional may have been able to take him down, if he was close enough - <10 feet. Still, there are just some times when there is no way to win.