He had already shown his demeanor with the spray-paint and the gun just confirmed it. If the guard wasn't in the room until after the shooting started that is bordering criminal. Waiting to see if he might shoot leads to more deaths.
The fact he had zero skills was the fortunate thing.
The area this occurred in would generally be considered low risk. I’m surprised they had an armed guard at all.
Its my guess he wasn’t in the room since the gunman kicked the audience out of the room prior to his shooting.
The desk did a good job of stopping the rounds aimed at the people. That was also a factor.