The real question is why was this girl mentally unstable and it wasn't recognized? The gun wasn't and isn't the problem, mentally unstable, dangerous people are. Maybe we should focus on the reasons for that rather than try and keep certain tools away from them. Banning guns won't solve unstable people trying and even succeeding in murdering people.
The world is full of mentally unstable people.
Several right here on this board.
You'll never cure it or control it.
The only thing you can do is take away the tools that make killing easy.
Guns make killing easy.