The details emerged when Ryan Grantham, 24, appeared in court this week for sentencing for the murder of Barbara Waite in April 2020, CBC reported.
Grantham, who pleaded guilty to charges of second-degree murder in March, admitted shooting his mother in the back of the head as she played piano in their townhouse in Squamish, north of Vancouver.
The next day, the former child star packed a car with three guns, ammunition, 12 Molotov cocktails, camping supplies and a Google map with directions to Mr Trudeau’s Rideau Cottage residence, prosecutors told the court.
The CBC reported that Grantham had written in a diary about his intentions to kill Mr Trudeau and in a statement to police.
He drove about 200 kilometres east to the town of Hope before turning around, and contemplated committing an act of mass violence at Vancouver’s Lions Gate Bridge or at Simon Fraser University, where he was was studying.
Instead, he drove to a Vancouver police station where he told an officer: “I killed my mother.”
Grantham, who started acting professionally at the age of nine, played the supporting role of Jeffrey Augustine in one episode of the CW show Riverdale in 2019. He has also appeared in TV show Supernatural and 2010 film Dairy of a Wimpy Kid.
The court heard that the actor had been struggling with anxiety, self-loathing and depression, and had expressed a desire to commit suicide or violence against others in the months leading up to his mother’s murder.
He carried out a walk-through of the murder by secretly approaching his mother from behind with a gun, the court was told.
The court was shown footage captured on a Go-Pro camera of one of his walk-throughs.
In another taken after the killing, he films his mother’s body while saying: “I shot her in the back of the head. In the moments after, she would have known it was me.”
Grantham told police in a statement that he spent the hours after the murder smoking marijuana, drinking beer, and trying to make molotov cocktails.
The next day he arranged candles around his mother’s body before setting off in his car.
His sister Lisa Grantham discovered their mother’s body that evening after repeated calls and messages went unanswered.
In a victim impact statement, she said that her life had been “devastated” by the murder, and that her brother was a dangerous person, the CBC reported.
On day two of the sentencing hearing, the court heard Grantham was suffering from intense clinical depression in the months leading up to his mother’s death.
Two psychiatric reports commissioned after the murder found that he had a cannabis use disorder after years of heavy use, but was able to maintain an outward appearance that he was functioning normally.
According to the two reports, Grantham told experts he wanted to kill his mother to spare her from witnessing the violence he was planning to commit.
Prosecutor Michaela Donnelly told the court that the 24-year-old had committed a “heartbreaking breach of trust”.
“Barbara Waite loved her son very much, was an excellent parent and in her mind had no reason to fear him,” Ms Donnelly said, according to the CBC.
Grantham was an experienced gun user who had been a member of a gun club. He had purchased the .22 calibre rifle in the months leading up to the homicide while aware of the homicidal feelings he was having, she said.
Prosecutors have asked for a minimum non-parole sentence of 17 to 18 years.
The sentencing hearing is continuing.