When cop killer Mitchell Barbieri was a young teen, his mother Fiona was a high-flying executive.
But by the time he was about 15 years old, his mother's descent into paranoia began and their life became "bizarre and quite extraordinary".
"The crazier she got, the more protective Mitchell became," neighbour Virginia Stern said in a statement tendered in the NSW Supreme Court.
This story of a mother's mental illness and the impact on her son needed to be told in order to understand why a 19-year-old with no criminal conviction ended up murdering Detective Inspector Bryson Anderson on December 6, 2012, defence barrister Robert Sutherland SC said.
The much-loved policeman died after he was fatally stabbed by Mitchell following a siege at the Barbieri's rural Oakville property in Sydney's northwest.
Mitchell has pleaded guilty to murder while his mother Fiona, 46, has pleaded guilty to manslaughter on the basis of substantial impairment.
Speaking at their sentence hearing on Tuesday, Mr Sutherland said Fiona was a senior executive at American Express.
But by about 2008, she experienced late onset paranoid schizophrenia and began believing the credit card company "was ruining her life and that the devil was trying to kill her".
She believed she was being persecuted by police and political leaders, the court has heard.
"It started to feel like they were both paranoid, like they believed the whole world was against them," local Renee Moorley told police.
In April 2011, a mental health worker who visited the Barbieri property reported Mitchell was beginning to "mimic" his mother.
By 2011, there was no electricity on the property, with the mother and son eating dinner by candlelight and charging their phones and laptops through a motor vehicle.
"(Their life) was bizarre, odd, unusual and quite extraordinary," Mr Sutherland said.
The crown concedes that on the day of Det Insp Anderson's death, Fiona's delusions had been transferred to her son.
Due to this impairment, the 21-year-old will not be subject to the mandatory life sentence reserved for people who have murdered police.
The Barbieris are due to be sentenced on December 18.