Nathan Gann, a clean-cut honors student who attended the University of Arizona, was a self-described computer geek. During the summer of 2007, 19-year-old Gann was busy blogging and posting videos of himself, which were goofy but showed a sense of promise and ambition, all over YouTube.
"I love the stars, ever since I was a child," he wrote on his MySpace page. "Because of them I have always wanted to reach for the stars. My dream goal is to be an astronaut."
Nathan's younger sister, 17-year old Brae Hansen, was equally driven. A high school honors student, who had recently returned from a church trip performing community service in Puerto Rico, she embraced life.
Watch the full story on "Primetime Crime" Wednesday at 10 p.m. ET
But in July 2007, disaster struck. At first it looked like a home robbery gone wrong: Brae Hansen told the 911 operator that a masked intruder had shot her 63-year-old stepfather, Tim MacNeil, in their San Diego home.
But as police investigated the July 2007 shooting death, two unlikely suspects emerged -- MacNeil's own stepchildren.
Why would these teens want their stepfather dead? The evidence didn't seem to add up. Brae reportedly adored her stepfather, whom relatives said she always affectionately called "Daddy."
Tim MacNeil had met and married their mother, Doreen Hansen, a schoolteacher, when Nathan and Brae were 7 and 5 years old. Erin MacNeil, Tim MacNeil's daughter from a previous marriage, told ABC News' Mary Fulginiti that Brae and Nathan were part of the family.
"They were his kids," she said. "He treated them just like any other ... just like he treated me."
However, Brae and Nathan were two kids who had spent their lives desperately trying to earn their mother's love.
"She was physically violent with him, hitting him ... with sticks," claimed Nathan Gann's attorney, Ricardo Garcia. She would, ridicule him. He would be in the bathroom ... and [she would] bring his sister in and point out his penis and ridicule him about it in front of his sister."
According to Nathan, his mother's verbal threats were just as violent as the physical ridicule.
"She had threatened to kill me several times," he told ABC News' Mary Fuginiti in an April 2009 interview. "She had told me that she was gonna kill me and bury me in Mexico."
Brae reveals that she, too, was tormented by their mother's suicidal thoughts. "She would just say, you know, 'What would you do if I ... you know, drove off the Coronado Bridge? What would you do if, you know, I just burnt the house down?' And this is when I'm, you know, 10, 7."
In a childhood of chaos, Tim MacNeil appeared to provide an oasis of sanity for the kids.
"He'd say, 'Doreen, that's enough'," Nathan told ABC News. And according to Brae, MacNeil was the one person in the household she believed she could trust.
When Nathan Gann was 12, MacNeil arranged for him to go live with his grandmother in Arizona to escape the turmoil at home. But after he left, Nathan's name was rarely mentioned, according to MacNeil's daughter, Erin.
"It was really weird. ... It was never spoken about to me. He was just gone," she said.