The occasional gun massacres are mostly due to the intersection of
two rights: the right to own guns, and the right to not be involuntarily committed for mental illness if one has not committed a crime.
The former dates back to the founding of the country; the latter is surprisingly recent. One seasonal movie,
Miracle on 34th Street, was released in (and is set close to) 1947. Though the movie has its share of fantasy elements, one aspect that viewers of the time did not find fantastic was the prospect of a lucid adult being committed by a court due to exhibiting delusions, with no solid proof of his actually presenting a danger to anyone. That didn't really change much, either, until (IIRC) the 1970s.
I wouldn't want to go back to those days, exactly. (For one thing, I'd miss many of my American JREF friends.

) But currently, parents and associates can report to the police that a person is collecting guns, describing violent fantasies, exhibiting mood swings and altered personality, and the police can take no action. By the time an actionable crime has been committed, it's likely to be so serious in nature (e.g. making "terroristic" threats if not an actual killing) that the person's mental health is now a secondary issue.
A disarmed populace can be abused and exploited by a tyrannical government. So can a system of involuntary mental health intervention, as was clearly shown by the former Soviet Union. Weighed against that, people do (and always have; this is not a new phenomenon) sometimes become mentally ill and flip out with guns, if they're not locked up and if they have guns.
I don't know what the right answer is. Some degree of risk seems unavoidable; all we can do is shift risk from one type to another.
For instance, what can we do about those lethal metal child killing machines, that Americans not only insist on having as much access to as possible, not only insist they couldn't live without, but also romanticize to the point of fetishism? (I'm talking, of course, about automobiles.)
Respectfully,
Myriad