The number of AR-15 style rifles manufactured exploded after the ban. I suspect they got popular among the gun lovers crowd. The demand likely made them more accessible to the crazies.
When I think common sense gun laws, I think keeping high powered, high capacity, high fire rate, accurate rifles out of the hands of people capable of killing kids is sensible. You can tell from those Instagram messages, the majority of people who knew that kid had suspicions that he was capable of a mass killing. Common sense says to do everything possible to keep a gun out of his hand.
The gun is inanimate. It can't be the cause. But damn, it can't hurt to use a little common sense and say no, we can't sell you that gun. Or if he takes it from mom, maybe mom, you can't have that gun.