A clearer picture of Elliot Rodger, the young man who went on a killing spree that resulted in the deaths of seven people, including himself, in southern California over the weekend, is beginning to emerge.

Police say Rodger, 22, stabbed three people to death in his apartment Friday night, before setting out on a shooting rampage in the community of Isla Vista in Santa Barbara, killing three others and then himself. Officers said 13 other people were injured.

Before the killings, Rodger posted a video to YouTube Friday, in which he detailed his plans for what he called "the day of retribution." The video he posted has been taken down, but copies of the video can still be found on the site.

"Tomorrow is the day of retribution, the day in which I will have my revenge against humanity, against all of you," Rodger says calmly, addressing a camera from the front seat of a car.

In the video Rodger says that he is seeking revenge for a life filled with "loneliness, rejection and unfulfilled desires."

He talks at length at how girls have rejected him, while giving their affection to other men. He then goes on to describe his plan to kill girls at a sorority on the campus of the University of California Santa Barbara, and then continue his slaughter on the streets of Isla Vista.

The video was posted on a YouTube channel that features several other videos he made. In many of them, which have names including "Why do girls hate me so much?" and "Life is so unfair because girls don't want me," he talks more about the rejection and pain he says he's endured at the hands of others. Rodger’s YouTube channel is still available on the site as of Monday morning.

Rodger also wrote a 140-page manifesto titled "My Twisted World," in which he explores the same topic, and shares details of his life growing up as the son of Peter Rodger, an assistant film director who worked on "The Hunger Games" movie.

Brad Garrett, a criminal profiler and former FBI negotiator, said Rodger’s videos and manifesto suggest he was thinking about the attack for a number of years.

"His anger and rage, disappointment and humiliation, have been escalating and by the time he reached his early 20s he basically couldn't take it anymore," Garrett told CTV's Canada AM on Monday.

Garrett, who is not involved in the police investigation in Isla Vista, noted that Rodger expressed resentment at multiple groups of people, including women, other men, and even his family.

"As his world became narrower and narrower and he eliminated people -- girls, boys, roommates, even possibly his family -- that if they would only change and take care of him and show attention to him, then things would change," he said.

"But of course when you develop that sort of reality around you it's only going to get worse. And of course it did."

He added that Rodger likely posted the video of his plans before he exacted them, in a bid to reclaim control.

"This is all about power, this is about taking charge of his life," Garrett said. "From his very deluded view of the world, he felt powerless. As he moved closer and closer to this act he started to feel more empowered."

Garrett noted that in the video, Rodger looks "relaxed" and speaks in a "straightforward" manner, demonstrating that he believed he was finally in control.

He said Rodger likely believed, at that moment, that by killing his enemies he would finally get the attention he sought for so long.

"This is about taking charge and telling the rest of the world 'I'm now in charge and because you have harmed me I'm going to remove you,'" Garrett said of Rodger's probable motivation.

Rodger's parents said that on Friday night before the killings, they had learned of their son's manifesto and YouTube video and raced to Isla Vista to intervene. But by the time they arrived, it was too late.

Over the weekend it was revealed that Rodger's family had alerted police in the past over concerns for their son, who they said was under the care of therapists. Police had checked up on Rodger to assess his mental health, but concluded that he posed no risk.

With files from The Associated Press