Mental illness
It's indeed very difficult to deal with mental illness.
But people with mentall illness are all over the world.
They will try whatever they can find at short range and quickly.
It could be a knife, a car or something else.
Guns are far too easily accessible in the US and guns can hurt many people from a distance in a very short while.
(I slept in my mom's cousin's room in 1976 and my grand uncle had a collection of guns hanging on the wall and he asked me not to ever open the shutter for fear anybody could see them from outside. My thought was: well, this is a worry I had no idea could exist!).