If more in that theater had been packing, the slaughter by this nut would have been cut very short. If he’d even dared to begin it. For that coward, it was like shooting the proverbial fish in a barrel.
I do think the shooter was worried about someone exercising his 2nd Amendment right to shoot back, hence his use of body armor and tear gas and shooting from the front where no one was naturally behind him. Unlike most of these monsters, he did not intend to die during the event.