Sorry, but this is not accurate.
The shooter approached, drew a weapon and was immediately tackled and/or shot. If everybody within shooting distance had fired at him, the only likely result would have been a lot more friendly fire casualties.
Surprise attacks, by definition, almost always get off a few shots before defenders can respond.
Didn't you see The Matrix?
Moving Security to the front doors would only result in the attacks taking place outside rather than inside. Anywhere you locate Security, it will result in a large, stationary, vulnerable crowd.