Benjamin Coman held delusions that his ex-partner's unborn baby was not his when he killed the mother-of-two at their former family home.
He was also paranoid supermarkets were poisoning his food, he was being spied on and a client was trying to defraud him of money, a Melbourne court has been told.
Coman, 31, faced the second day of a pre-sentence hearing on Tuesday after he pleaded guilty to the October 2021 murder of Michelle Darragh, 32, inside the home they once shared with their children in Bayswater North.
Ms Darragh was trying to leave Coman and had gone to the house on October 9 to collect some belongings.
When she didn't return home to her parents to feed and bathe her kids, her father became concerned and went to the house, where he found his daughter's lifeless body.
Ms Darragh died from a large stab wound to her back.
Forensic psychologist Andrew Carroll was called to give evidence on Tuesday about Coman's mental health leading up to the murder.
His condition deteriorated in the weeks prior and he was admitted to Maroondah Mental Health ward in mid-September for psychiatric help after he tried to take his own life.
Following years of heavy drug use, Dr Carroll said Coman developed psychotic depression, which included paranoid thoughts and "heavy delusions" about infidelity.
"Objectively Mr Coman had no reason to be angry with Ms Darragh. He did feel angry at the time. His thinking the child was the product of an affair, was a delusional belief," he told the Supreme Court.
"He came to believe his partner could read his mind through some device that had been inserted into his head through an operation a few years prior."
Earlier, Ms Darragh's siblings read statements to the court and addressed Coman, who kept his eyes closed while he sat in the dock.
"Don't be such a coward, open your eyes," her brother James Darragh said to Coman.
Samantha Darragh described the heartbreaking 10-second call she received from her mother on the day her sister was killed.
"I think about mum screaming down the phone to me... 'Ben's killed Michelle,'" she said.
"We trusted Ben, we trusted him with 'Shell. For him to take the life of someone who he was supposed to love and protect just shows everyone who he really is - a monster."
Coman kept his eyes closed until Ms Darragh's niece read out her statement.
He cried as the young girl explained how Coman had comforted her at a family member's funeral before he killed her aunty.
"The fact someone could do this to my beautiful aunty makes me sick," she said.
The hearing continues before Justice Andrew Tinney.
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