when the 2nd Amendment was ratified there were no doubt some mentally ill citizens. But in those days of muskets it was a bit of a challenge for someone like that to get hold of a musket and bullets and gunpowder and everything needed to fire the thing. Then putting it all together correctly so it would fire was another challenge. It's not something a person would tend to do in a mental breakdown or fit of rage. Even if all went well it required going through the process of reloading after every shot. "Mass shootings" were basically an impossibility in the musket days.

250 years later it's virtually effortless for almost anybody to load up a semi-automatic rifle or a handgun and spray a school with bullets.

The 2nd Amendment should have long ago been adapted to account for the radical changes in gun technology. There will always be people with mental health issues.