and I'll keep my scutum in my kid's diaper bag, next to the diaper cream and cheddar goldfish.

The problem here is that Trolley Square and Virginia Tech are mass murders by deranged people. There were warning signs that were ignored or missed, which is the only way to really prevent these kinds of attacks.
In Trolley Square, it was indeed fortunate that an off-duty police officer ignored the posted signs against weapons and carried a pistol into the building. But I'm not buying the sword theory. Bombs, maybe. This Korean guy at Virginia Tech had the time and drive to make a suicide vest, a la Iraq, and turn himself and a bunch of other people into chunky salsa.
Generally speaking, I'd rather be attacked by a deranged maniac with a gun than a bomb. At least with a gun, people start running when they hear gunshots. Nearby police or armed citizens have a chance to intervene, scared civilians have a chance to run and/or take cover. A bomb explodes far to fast for anybody to escape the blast and fragments.
The vast majority of killings in this country not mass shootings, they are a direct result of our failed war on drugs, chronic poverty, and social and economic injustice. They are gang members killing each other, they are career criminals killing innocent store clerks, murdering people in their homes, and leaving pedestrians dead on the sidewalks after a mugging. Some of them are crimes of passion or revenge. One or two people at a time, 40 per day.
Addressing the majority of our resources on daily murders instead of trying to stop relatively rare mass killings will save far more lives and help far more people. Better mental-health care just might help to, yanno?