Prince Fahnbulleh, 24, attended his former partner's flat in Sydney's west after their relationship ended to retrieve clothes she left outside for him in June 2023.
He instead entered her home through a window and found Yohana Angok asleep in bed with her.
Fahnbulleh ran to the kitchen, grabbed a knife and murdered Mr Angok with a single stab to the heart, who had by that time awoken to the ruckus and attempted to defend himself with a pillow.
"The killing was not planned or premeditated... the killing was a spontaneous, jealous response," Supreme Court Justice Deborah Sweeney said on Thursday.
"Angkok was 24 ... killed a day before his birthday ... a much loved son and brother to his nine siblings," she said.
"He was defenceless".
Fahnbulleh appeared in court via video link from prison where he has remained since his arrest days after the murder.
The 27-year-old's head remained down and shook from side to side as Justice Sweeney described his "obsessive jealousy".
It included texts Fahnbulleh sent Mr Angok days before the murder.
"I'll find you soon or later," he messaged him alongside a picture of a handgun.
"I'm not going to stop 'til you get shot in your head think I'm playing," Fahnbulleh wrote in another message to his soon-to-be victim.
During his murder trial in May, the jury heard from his friend Adrian Shillingsworth who dropped Fahnbulleh off at the flat.
The killer returned to the car two minutes later.
"He just told me to drive", Mr Shillingsworth told the jury.
"I just saw blood and I was panicking," he recalled.
After dropping Fahnbulleh at home, Mr Shillingsworth walked to the nearest police station and told officers what happened, electing not to drive because of bloodstains on his passenger seat, dashboard and roof lining.
Fahnbulleh meanwhile fled to Canberra but his attempted escape failed.
Police picked him up after he was involved in a minor car accident four days later and extradited him back to Sydney.
The victim's DNA was found on swabs taken from Mr Shillingsworth's Ford Falcon door, seatbelt buckle and sun visor.
Fahnbulleh argued self-defence during the 10-day murder trial and claimed Mr Angkok had a cheese knife on him.
It did not convince the jury, which found him guilty after an hour of deliberations.
Justice Sweeney handed him a head sentence of 23 years in jail, set to expire in June 2046 with time already served.
He will be eligible for parole in June 2040.
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