Humans are not robots, these sorts of people are assuming that lead is going to only be flying one way. Even among professionals, when ranges go two way marksmanship goes to crap. Kevlar only keeps the round from penetrating, it doesn't do anything about Newtons laws of motion. What effect that will have on a crazy person is anyone's guess, but I can't think of anything that would make things worse, they might panic and run away, their careful aiming might give way to spraying.
Exactly. It's not like he had an hour long shoot out with police in the style of the North Hollywood robbers before he was taken. When confronted by police, he gave up. He might have been injured or given up if confronted by an armed movie goer.
An armed movie goer would have to make a quick decision to shoot, hunker down and shoot later or try to escape based upon what they could tell about the attack. Some people have already pointed out that a defender might not know about the body armor until they started shooting back. There are no good answers in a situation this so maybe a defender has to trust luck and any training to be in their favor.
I don't know if there were any concealed carry permit holders in the audience that had left their firearms behind as the theater's policy required but I would imagine that an armed off duty police officer might have made a difference for some of the victims.