I don’t particularly care what you think about lawful carry, the fact remains that if you want to keep casualties to a minimum in an active shooter situation it’s absolutely essential they are engaged immediately, not minutes later after police can arrive, assess the situation, come up with a plan of action, and move to interdict the shooter.
The average police response time to a shooting is over 5 minutes in the US and that’s from the time the call is completed.
Over 95% of school shootings are over in 5 minutes or less from the time the first shot is fired and over 90% of the casualties will occur in the first 3 minutes.
As for your example, had he been engaged in the same timely manner the math tells us there would have been no more than 2 dead including himself and most likely no other casualties at all.
There is no possible way on earth to keep the hands out of would be active or mass shooters but we do know the most effective ways of stopping them once they begin.