Prince Papay Fahnbulleh, 27, knifed 25-year-old Yohana Angok after seeing him in bed with Janika Bevin, his on-again off-again girlfriend of seven years, in June 2023.
He has pleaded not guilty to Mr Angok's murder, saying he acted in self-defence.
But crown prosecutor Mark Hay on Wednesday told a NSW Supreme Court jury the stabbing was the product of a jealous rage.
"On seeing his ex-girlfriend ... in bed with another man, (he) ran to the kitchen, returned with the intention to stab and, the crown case is, to kill or cause grievous bodily harm to Mr Angok", he said.
On the day of the killing, the prosecution alleges Fahnbulleh had arranged to collect clothes from his ex-girlfriend's western Sydney apartment, which she left for him outside.
But Mr Hay told the jury the accused ignored the clothes and entered the unit via a staircase in the building's backyard "uninvited and unannounced".
"Those clothes did not get touched", he said.
"The accused came to the property over the side gate, up the stairs and in."
On entering, Fahnbulleh discovered the pair in the bedroom, ran to the kitchen, grabbed a knife and stabbed Mr Angok, before fleeing through a window and over the building's garage roof, Mr Hay said.
He was charged with murder four days afterwards, after being involved in a minor car accident in Canberra.
During his opening statement, Mr Hay also showed the jury text messages Fahnbulleh sent to Mr Angok in the days before the killing.
"Send me your address, stop being a bitch, I'll find you sooner or later", one read.
Three days before the murder, he hijacked Ms Bevin's phone after seeing messages between her and Mr Angok on it.
On taking the phone, he told his ex-partner "(Yohana Angok) yeah...he's dead, he's dead, watch me", court documents read out on Thursday show.
He then used that phone to send Mr Angok more texts.
"Were (sic) you at I'm not going to stop till you get shot in your head think I'm playing", Mr Hay read to the jurors.
Those texts included a photo Farnbulleh allegedly sent the deceased of himself brandishing a handgun.
In his opening statement, defence barrister Scott Fraser SC told the jurors "a first impression might have been to wonder what we're doing here" after seeing Fahnbulleh's texts.
"This trial is, members of the jury, not a whodunnit", he said.
But Mr Fraser asked the jurors to consider how Fahnbulleh's threatening messages might have made the victim react to seeing Ms Bevin's ex-boyfriend in the bedroom that day.
"Could (the defendant's messages) in fact ... explain the actions of the deceased prior to him being stabbed?" Mr Fraser posited.
He said Mr Angok was armed and instigated the violence himself.
The trial continues.
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