The older brother of the perpetrator said his parents "lost control" of their son and he feared his family member would fatally harm a member of their household, a hearing has heard.
The attacker, aged 17 at the time, murdered three kids at a pop music-inspired dance workshop on July 29, 2024.
The official investigation, conducted at the venue, was told the family member he informed a friend there was a danger of his brother "taking action deadly."
The hearing was informed the brother messaged his contact over the Discord messaging app: "The fights are frightening because of the risk of loss of life."
Dion said his relative was similar to the sociopathic murderer in the picture the Coen brothers film.
Increasing Conflict
The inquiry heard he was found to have a health issue at the twelve years old which resulted in him using a wheelchair and his parents helping him more.
When asked by Richard Boyle, attorney, if that altered their dynamic he replied: "Strain developed that came about."
Dion concurred his brother seemed to dislike this shift.
He testified the investigation that after the relatives left the Welsh capital to Southport his brother's moods deteriorated and he would have "aggressive episodes."
The inquiry heard the brother became "progressively concerned" of his sibling, who would hit him regularly.
He said: "I needed to be careful if I did speak to him because every argument could escalate into an confrontation."
'Genuine concerns'
Dion testified his relative became much more aggressive after he was removed from the educational institution in the local area, in October 2019.
He left for university in 2022 and the two communicated rarely "as he was unaccustomed to having me around."
The hearing was informed of a note he wrote to a acquaintance on the service in which he mentioned his relative was angered by him having conversations after dark because of the limited soundproofing in their residence.
He also told his friend there was a possibility of "him doing something potentially fatal."
He said: "Confrontations are frightening because of the danger of loss of life."
Mr Boyle asked: "You had serious fears that your relative would murder a person of your relatives?"
The brother responded: "If things escalated to that point."
Mr Boyle questioned how his parents dealt with the perpetrator's aggression.
"It didn't make sense to seek to penalize him," he stated. "Also, there was a serious threat in doing so.
"We said there was a general risk to safety in general conversation, if you attempt to challenge him... it wouldn't be responded to well."
When the lawyer inquired if his mother and father had lost control of his sibling, the brother acknowledged.
Questioned regarding his sibling being similar to a character from the movie the thriller, the brother said: "It was said that character's intended as a psychopath and that's why I chose that description there."
The brother said the final contact he had with his brother was in the summer of 2023.
He acknowledged that their family told the perpetrator to say goodbye to him as he was departing to visit his friends and his brother hurled an object at him but the exit was secured before it made contact.
The inquiry continues.
