The victim, Jeff Hall, was an out-of-work plumber who as regional leader of the National Socialist Movement headed rallies at a synagogue and a day labor site.
In opening statements in Juvenile Court, Riverside County prosecutor Michael Soccio said the now-12-year-old boy wanted to kill his father because of a history of domestic violence.
Soccio dismissed the notion that the neo-Nazi beliefs of Hall contributed to the son's penchant for violence, as the defense maintains.
"You'll learn that (the child) would have shot his father even if he'd been a member of the Peace and Freedom Party. It made no difference," he said.
Soccio said the boy told his younger sister the day before the May 2011 killing that he planned to shoot their father and she told him not to.
The boy saw an opportunity on the evening of the killing when his father came home from a party but was locked out and had to get in the house by crawling through a window, the prosecutor said. Hall fell asleep on the couch, and the boy got a gun and shot him while standing by his shoulder, Soccio said.
"He held the gun about a foot away and as he explained, he took four fingers and put them into the trigger and pulled the trigger back and the gun discharged," Soccio said, showing images of a bloodied victim on the couch covered by a blue blanket.
Defense attorney Matthew Hardy countered in his opening that his client had grown up in an abusive and violent environment that led him to believe it was good and right to kill people who were a threat to one's safety and the safety of one's family.
The boy's father taught him to shoot guns, took him to neo-Nazi rallies and once took him to the Mexican border to teach him how to protect his country from illegal immigrants, Hardy said.
"If you were going to create a monster, if you were going to create a killer, what would you do?" he said. "You'd put him in a house where there's domestic violence, child abuse, racism."
Hardy also alleged that the boy's stepmother Krista McCrary, who is expected to testify, manipulated the boy into killing Hall because Hall was going to leave her for another woman. Hall sent her text messages that night telling her he would divorce her and had left a party with another woman, Hardy said.
McCrary sat in on the child's interviews with police and psychiatrists after the shooting, he said, and she lied to investigators herself.
The prosecutor said previously that the boy's history included being expelled from school for violence at age 5, long before Hall became a neo-Nazi. The school violence included choking a teacher with a telephone cord.
The Associated Press is not identifying the boy — who is not charged as an adult — because of his age.
Hall, 32, who said he believed in a white breakaway nation, ran for a seat on the local water board in 2010 in a move that disturbed many residents in the recession-battered suburbs southeast of Los Angeles. The day before his death, he held a meeting of the neo-Nazi group at his home.
Last year, the boy — the oldest of Hall's five children — told investigators he went downstairs and shot his father before returning upstairs and hiding the gun under his bed, according to court documents. He told authorities he thought his father was going to leave his stepmother, and he didn't want the family to split up, Soccio said previously.
The boy's stepmother told authorities that Hall had hit, kicked and yelled at his son for being too loud or getting in the way. Hall and the boy's biological mother had previously slugged through a divorce and custody dispute in which each had accused the other of child abuse.
Kathleen M. Heide, a professor at the University of South Florida in Tampa who wrote "Why Kids Kill Parents," said children 10 and under rarely kill their parents and that only 16 such cases were documented between 1996 and 2007. Heide also said parenting and home life would undoubtedly play a role in the case.
"It would be inaccurate to say who the child's parents are is superfluous," she said. "That is going to have an effect on how the child grows up, on the values that child learns, on problem solving abilities, so all of that is relevant."
If a judge finds the boy murdered Hall, he could be held in state custody until he is 23 years old, said Bill Sessa, spokesman for California's Department of Corrections and Rehabilitation.
The state currently houses fewer than 900 juveniles.
"We don't have anybody that young," Sessa said. "We have had 12-year-olds in the past, but it's rare."