John Steele and his son David were looking through photographs of their former partner and mother, Jennifer Cole, who had died from cancer a day earlier.
Soon after, David Steele was dead on the floor from a single catastrophic stab wound to his neck.
A Supreme Court justice described the case as "unusual and very tragic" on Tuesday as she sentenced Steele to six years and nine months in jail after the 76-year-old pleaded guilty to manslaughter.
The two men were living together on April 28, 2022, at a property in South West Rocks, on NSW's mid north coast.
A day before, Ms Cole had died from cancer at Kempsey Hospital.
After going out to dinner together along with Steele's daughter, Aimee Cicciarello, the trio returned home and began looking at family photos and talking about Ms Cole.
Steele had separated from Ms Cole many years earlier after having been married for 17 years and raising three children together, but remained on good terms.
An argument developed when David asserted Steele and Ms Cole were having an argument in one of the photographs.
The dispute escalated to the point where Steele went to the kitchen and grabbed a knife, saying words to the effect "if you come near me, I'll kill you".
The 39-year-old moved quickly towards his father, who stabbed him once in the neck.
When paramedics arrived, they found David lying face up on the dining room floor surrounded by blood, with Steele leaning over him.
As he was moved away, Steele was overheard saying to Ms Cicciarello: "Don't worry. I did it. It's okay."
He later told police he couldn't clearly remember the event but was under the impression his son intended to push or knock him over.
"I miss my son immensely and wish I could turn back time, but I can't," Steele wrote in a letter to the court.
In sentencing Steele, Justice Jennifer Davies described the event as "unusual and very tragic" and noted there were other options available to diffuse the situation short of stabbing his son in the neck.
"What could have possessed the offender, who apparently loved his son, and who on every account was a fine, upstanding member of the community with no history of violence, to grab a lethal knife, threaten to kill his son, and then to do so, all because at the end of an argument the son came running towards him?" she wrote in her judgment.
"It did not need to happen, but it is something the offender will have to live with for the rest of his life."
Steele will be eligible for release on October 27, 2025, with a non-parole period of three years and six months backdated to the time of his arrest.
