Jennifer Petelczyc, 59, and her daughter, 18-year-old Gretl, were killed in their Perth home on May 24, 2024.
Their killer, Mark James Bombara, 63, then turned the gun on himself as police arrived.
An inquest has heard Bombara had been searching for his estranged wife Rowena, from whom he separated a month earlier, but she wasn't at the home.
The break-up and a series of strokes had significantly challenged Bombara's "sense of self" as a successful businessman, a forensic psychiatrist told the inquest on Friday.
Bombara was depressed, and coupled with his obsessive and narcissistic traits and rigid thinking, the gunman wasn't coping with the changes to his life, Mark Hall said.
"He had already adopted a nihilistic view of the future, and he believed this was the end for him," Dr Hall told the WA Coroners Court on Friday.
Dr Hall said Bombara's personality traits contributed to the way he perceived what was happening in his life and "how he viewed his prospects and remedy" before the Petelczyc murders
"But it's not a single explanation," he said.
Bombara was also exhibiting signs of paranoia, accusing Rowena of engaging in "medical voodoo" when she gave him an anti-inflammatory to help his gut condition.
His mental health may have further unravelled when police arrived at the Bombara family home in early April to help Rowena and their daughter Ariel move out after they voiced concerns for their safety.
"He was a very emotionally constricted man in many ways, and the notion he was perceived as violent was tacit permission to feel that way," Dr Hall said.
A visit to one of his and Rowena's favourite cafes in the hours before he went to her best friend's home with a gun could have tipped Bombara over the edge to the "point of no return", Dr Hall said.
"I don't necessarily think that he thought he wasn't coming back to the community," he said.
"It was entirely possible it was another thing to tick off his list before death came to him.
"He bought a boat, which was his life dream ... and now he was addressing the task to his financial legacy that he thought may come from Rowena's settlement in the divorce."
Dr Hall said it was unlikely Bombara was thinking "very far ahead" and may not have intended to kill himself.
He said if police hadn't arrived at the Petelczyc's home, he may have "regrouped" and continued searching for Rowena.
Constable Paul Czirr was one of the police officers sent to the Bombara family home in April after Rowena and Ariel raised concerns the gunman's weapon collection.
He only spoke to Bombara and said it was a "nice, clean home", and there were no red flags.
Const Czirr and his partner didn't assess whether Bombara was in a fit state to possess firearms.
"Looking back, I can see I was being manipulated," he said.
"I went in to build rapport ... I was essentially there to provide security."
The inquest continues.
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