He bailed when the shooting started, then tweeted that he was ok. My impression was that he was a total coward.
The performers on stage probably had a pretty good view of the initial carnage, when every shot was finding a mark. If you saw someone take a bullet to the head right in front of you what would you do? For that matter, Jason Aldean may have been the initial target, for effect, but the distance allowed the rounds to fall a bit and strike the fans instead.