The attacker did not have his weapons legally. Let's get that out there. He murdered his mother with her own guns, which he then took to kill a bunch of elementary school children who could not shoot back (schools are gun free zones, last I checked).
Laws cannot and will not deter criminals, while over-broad gun control legislation will force some groups to sue based off of the 2nd amendment to the US Constitution.
Criminals do not, by definition, care about laws. There are some things that most criminals will not do, but that does not include theft, smuggling, etc.
Criminals, therefore, will have access to guns so long as guns are made. There will always be third parties willing to smuggle guns into the US in order to make a profit.
Smuggling is already illegal.
Murdering someone is already illegal.
Owning a weapon is not illegal. There is a reason for this. Guns don't murder people. People murder people, sometimes with guns, sometimes with baseball bats.
Yet no one wants to make baseball bats illegal (except on planes).
It is my argument, however, that the only weapon that can be outlawed to protect the innocent is the human brain. Ban a weapon? The human brain will eventually be able to come up with a way around the ban. Even if it is only otherwise innocuous objects. Blunt instruments are an example (backpacks with textbooks, laptop computers). Anything can be dangerous, given the right (or wrong) mindset.
In any case, the shooter was a criminal. He murdered his mother, illegally obtained her weapons (stolen from a dead person), and illegally took those firearms to a location where firearms are illegal.
At what point in that was he a law-abiding citizen (once he killed his mother)?
I will say that it is good that the shooter did not set fire to the building after turning off their water supply. That would have dramatically increased the death totals.
The water main(s) are cut off by homemade explosives, and the hypothetical terrorists set fire to the school. What happens if the terrorists only prevent people from leaving, and firemen from coming?
Do the children burn to death? How long does the school take to burn? How long does it take to remove the terrorists? Does the school have a backups for their sprinkler systems?
What scares me is that sometime one shooter will use his brain to increase the death totals. Or a group of terrorists will use their brains.
I would rather give every teacher a gun and teach them how to use it and prevent children from getting ahold of it than have a shooting where the victims cannot shoot back and kill the attacker(s). Yes, that would increase the potential for bystander deaths, but will that be less than the death total would otherwise be?
The Second Amendment to the US constitution is there for a reason. It is supposed to help prevent the government from becoming a totalitarian dictatorship by enabling the citizens to fight back. This is becoming more difficult, as gun laws disarm the citizens. Sometime in the future, it is possible that there will be amendments to the US Constitution removing the second amendment and making the US a dictatorship.
That is worrying.