Jesse Thompson, 19, was in the back seat of an SUV when he was fatally shot on the streets of Wyong, on the NSW Central Coast, in 2017.
At the time, the SUV was being pursued at high speeds by a ute driven by John Paul Evans, 57, with his son Keith Evans, 31, in the front passenger seat.
John Evans overtook the SUV and forced it to stop so the cars were nose-to-nose one metre apart, the NSW Supreme Court was told on Tuesday.
Justice Richard Weinstein said the position allowed his son to have the best shot when he discharged the shotgun at close range and hit Mr Thompson.
The Evans men intended to kill someone in the car with the firearm - which they knew was working and loaded - in an act of retribution, the judge found.
Keith Evans was looking for revenge after his girlfriend slept with one of Mr Thompson's friends, whom he assaulted in the lead-up to the murder.
But his father was looking for retaliation after two men smashed the glass front door of the family home following the assault.
"The offending was pitiless and cruel," Justice Weinstein said.
"At any time, the Evans family could have retreated and either reported the matter to police or headed home."
The judge expressed his condolences to Mr Thompson's family and friends, who described the teen as a "warm and loving young man".
"Jesse Thompson had everything to look forward to," he said.
"That he was murdered at such a young age is a tragedy not only for his family but also for the community."
In a letter of apology read in court, John Evans said he was "deeply sorry" for the hurt he caused Mr Thompson's family.
"They must hate me but not as much as I hate myself," he wrote.
"Not a day goes by that I'm not haunted by this loss of life … I relive it every day and ask myself: why?"
As the oldest person present and the driver of the ute, the court was told he could have ended the car chase.
But instead of diffusing the situation, Justice Weinstein found he inflamed it in an "alarming dereliction of parental responsibility".
Keith Evans denied he intended to fire the weapon that killed Mr Thompson but expressed sadness for the teenager's family.
"He died so young. It's something that never should have happened," he told a psychologist.
The father and son had an evolving insight into their deadly actions, Justice Weinstein found.
Keith Evans was sentenced to a maximum of 27.5 years behind bars for the murder.
With time served, he will be eligible for parole in November 2035.
His father was jailed for a maximum of 26 years and six months, which was backdated to when he was first taken into custody.
He will be eligible for release on in March 2034 when he is 68 years old.
The third person in the ute - a 25-year-old woman who cannot be identified because she was 17 at the time - has been jailed for more than 14 years after pleading guilty to Mr Thompson's murder.
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