I haven't seen any of the shooter's videos. But from what I've heard, most people were characterizing them as "weird" and not a rock-solid case that the is a danger to herself or others.

Given that, it sounds like very little would be able to prevent her from obtaining a firearm. She could likely walk right into a gun shop and pass a background check. As far as I know, she hasn't had a domestic violence conviction, involuntary commitment to a mental health facility, or anything else that would disqualify her from buying a gun legally.

Now, you may think that this is an issue in and of itself. But at least in the US, the right to bear arms is a natural right of the individual. It's protected by the constitution. And, there needs to be due process if you want to deny someone of their rights.