Constantly bullied and unable to fit in, he went through three high schools. In college, he tried to throw a girl off a ledge at a party — and was beaten up. (“I’m going to kill them,” he said to a neighbor afterward.) He finally retreated to some Internet sites that “drew sexually frustrated young men,” according to The Times.

Throughout, said one person who knew Rodger, “his mom did everything she could to help Elliot.” But what his parents never did was the one thing that might have prevented him from buying a gun: have him committed to a psychiatric facility. California’s tough gun laws notwithstanding, a background check would have caught him only if he had had in-patient mental health treatment, made a serious threat to an identifiable victim in the presence of a therapist, or had a criminal record. He had none of the above.

Should his parents have taken more steps to have him treated? Could they have? It is awfully hard to say, even in retrospect. On the one hand, there were plainly people who knew him who feared that he might someday harm others. On the other hand, those people weren’t psychiatrists. He was a loner, a misfit, whose parents were more fearful of how the world would treat their son than how their son would treat the world. And his mother, after all, did reach out for help, and the police responded and decided they had no cause to arrest him or even search his room, where his guns were hidden.

Once again, a mass killing has triggered calls for doing something to keep guns away from the mentally ill. And, once again, the realities of the situation convey how difficult a task that is. There are, after all, plenty of young, male, alienated loners — the now-standard description of mass shooters — but very few of them become killers.

And you can’t go around committing them all because a tiny handful might turn out to be killers. Indeed, the law is very clear on this point. In 1975, the Supreme Court ruled that nondangerous mentally ill people can’t be confined against their will if they can function without confinement. “In California, the bar is very high for people like Elliot,” said Dr. E. Fuller Torrey, who founded the Treatment Advocacy Center. In a sense, California’s commitment to freedom for the mentally ill conflicts with its background-check law.