What could have stopped him?
One bullet in the right place.
If you took every gun away from every person who legally owns them, there would still be a million guns in the hands of those who own them illegally and they would be available for sale in the street.
A crazy person has only to go on the internet to find ways to build bombs, and other ways to kill. Knives , baseball bats, automobiles,many thing will kill and some crazed POS will use them.
Taking away a person’s right to self-defense will not stop killings.
What could have stopped him? A $300.00 piece of bullet proof glass. With that in place, he cannot shoot out the glass to get into the school. The admin calls 911. The police come to the school and kill him before he can do any real damage.
If a school isn’t going to allow CC, then body armor the school. It doesn’t solve the problem, but it might just slow it down enough so the police have a chance.
Moreover, the price would go up and guns would start flowing into the country from Central and South America to meet the demand - one more profitable import for the Cartels.