I tell ya something that surprises me, the lack of people who are prepared to go on the offensive. If people stuck together, the shooter could have been overwhelmed. One guy charged him despite the odds. If others had followed, the shooter stupid face turd licker could have been taken down by an unarmed mob. Maybe some would have died anyway or at least have been wounded, but certainly more could have saved. As has been mentioned above, they're taught otherwise. To comply. To rules and attackers alike. It doesn't help when they're elected officials say things like attackers use guns because civilians have guns but if civilians put they're guns down then attackers would too.
I'd hope that if ever in that situation myself, we collectively would try to over take the attacker before it got to the point that people are lined up facing a wall. As a group, people need to move decisively and without hesitation. It's not a staring contest.