I've already mentioned what the owner should have done. It was a sell out so there were lots of people. He had cops in front for crowd control. So, he should have put ushers by the exit doors to make sure no one snuck in without paying. If someone left, they couldn't reenter. They should have been directed to leave through the lobby or entrance for phone calls, etc. I know at SEC football games, they check you for guns and alcohol before you can get in the gate, pockets and purses. Once you leave, you can't get back in. Of course it is 80-100,000 people and they have security at the gates big time. Any place with lots of people should have security.
Now, even if the guy had body armor on, a 357 round would probably have knocked him down, at least knocked the breath out of him so he could be taken down.
I've heard the shotgun had the most kills, not the AR-15. Lots of buckshot flying.