Elliot Rodger spent five years planning his "Day of Retribution" during which he intended to murder his 6-year-old brother whom he had once apparently saved from drowning.
The "virgin killer", who had never had a girlfriend, wrote in his manifesto that he was jealous because his younger brother had been signed up to act in television commercials.
In his manifesto, My Twisted World, Rodger wrote he had been teased by his "first crush" when he was a boy and it had "wounded me deeply".
Monette Moio, 20, now an aspiring model, was vilified by Rodger as an "evil b****" in the document and she is said to be distraught.
According to her father, John Moio, 75, a Hollywood stuntman, she barely recalls the killer whom she met at school when she was 10.
He said: "She doesn't even remember this guy. She has always been the most delicate kid you would want to meet.
It was just an illusion this kid built in his own mind."
Rodger, 22, also described how he planned to stab in the neck his stepmother, Soumaya, an actress. He intended to carry out the killings while his father, Peter Rodger, an assistant director of the first Hunger Games film, was away on business.
The student said he would murder his brother and stepmother a day before targeting people randomly in the university town of Isla Vista, California.
It was not clear why he altered the first part of his plan. At the time he carried it out on Saturday his father was at home, rather than away on business. Instead, Rodger stabbed to death three room-mates in his apartment, then went on a gun rampage around Isla Vista killing three more people and injuring 13 before shooting himself dead.
In the manifesto, he wrote: "I had an argument with Soumaya while I was visiting father's house. It started when she began to boast that my brother was recently signed by an agent to act in TV commercials.
"He will become a popular kid who gets all the girls. Girls will love him. He will become one of my enemies. That was the day that I decided I would have to kill him on the Day of Retribution. I will not allow the boy to surpass me at everything, to live the life I've always wanted."
Thousands of students attended a memorial service for the victims yesterday, as the University of California at Santa Barbara declared a day of mourning.
Students made 10,000 ribbons for the service at the university, which was still reeling from the killings.
"We are here because in this moment of loss there's a human desire to come together to reach out to one another for love and support," said University of California president Janet Napolitano.
"We are grieving together today as a family, as the community of the University of California and we will get through this," added the former US Secretary of Homeland Security.
The father of one of the victims pleaded with politicians to take action on gun control as the dead were honoured.
Christopher Michael-Martinez was one of the six students killed by Rodger.
Martinez's father, Richard, begged politicians to do something about the issue as he paid tribute to his son.
Addressing thousands of mourners at Harder Stadium on the university campus, Martinez said: "There haven't been any leaders on this issue. Yesterday I got a call from a congressperson to express their condolences for what had happened.
"I told that person I was not interested in a phone call of condolence; that was not acceptable until they went back to Congress and did something.
"They have done nothing and that is why Chris died. This situation has gone on far too long, we all know it."
Martinez received a standing ovation as he led the crowd in chants of "not one more" and encouraged people to send postcards bearing the same words to politician.