"They all display deluded thinking and a lot of rage about feeling so marginalised," said James Garbarino, a professor of psychology at Loyola University in Chicago.

Since mass killings are rare, scholars say there's no way to predict who has deadly intentions, let alone who will reach a breaking point and take action.

Past violence is a clue, but in Rodger's case, police did not see him as a threat to himself or others during a welfare check weeks before Friday night's rampage near the University of California in Santa Barbara that left six dead and 13 injured.

Rodger died of an apparently self-inflicted gunshot wound to the head after a shoot-out with sheriff's deputies.

Before he stabbed three male students in his apartment and cruised around in his black BMW firing at sorority girls and strangers, he left a trail of YouTube videos and a 140-page manifesto ranting against both women and couples and lamenting his lack of a sex life.

Rodger, a 22-year-old community college student and son of a Hollywood director, said he was a lonely and frustrated virgin.

"I'm sexually attracted to girls. But girls are not sexually attracted to me. And there's a major problem with that - a major problem.

"That's a problem that I intend to rectify. I in all my magnificence and power, I will not let this fly. It's an injustice that needs to be dealt with," Rodger said in one of the videos.

Recent mass shootings involved young men described as loners who had trouble fitting in. In July 2012, 24-year-old Holmes opened fire at a midnight screening of the Batman film , killing a dozen movie-goers. Five months later, 20-year-old Lanza shot 20 first-graders and six teachers at Sandy Hook Elementary School in the state of Connecticut.

Experts who study mass murderers say the vast majority of lonely and angry people don't commit violence, which makes it difficult to know who will snap.

James Alan Fox, a criminal justice professor at Northeastern University in Boston, said: "We can point to all the warning signs we missed. But they're yellow flags. They're not red flags until blood is spilled."

Before the killings, Rodger's Malaysian mother, Li Chin, became alarmed about bizarre videos he posted and alerted authorities in April. But Rodger was able to convince deputies he was not a risk to himself or others.

Family friend Simon Astaire said Rodger was "very much a boy of solitude" who spoke few words.

"At a Christmas party, I went out to get air and there he was standing alone. I apologised for disturbing his peace, and he said it was all right. I asked, 'How are you doing?' He said, 'I find things difficult.' I walked away thinking that he was very sad lonely boy," Astaire said.

In his writings, Rodger said he saw several therapists, but it's unclear what his problem was.

Experts say people with mental illness are generally not more violent than the rest of the population. A rare exception was Jared Loughner, who shot dead six people in the state of Arizona in 2011 in an attack that gravely injured Congresswoman Gabrielle Giffords. After his arrest, Loughner was diagnosed with schizophrenia.

On Sunday, several security guards stood watch outside the apartment building where Rodger lived. Memorials sprung up there, outside the sorority house where two women were shot nearby and at a deli where a male student was shot.

Garrett Schneider, a 22-year-old student, was touched by the tragedy, but said he won't view fellow students with more suspicion because of it. "I figure that people like this are far and few between," he said. "If you read his writing and look at his videos, it's obvious he's far out there."