Roy Tabalbag had been a "completely non-violent" and "non-aggressive" person before November 2013.
But after catching his partner naked in bed with another man, he picked up a large chef's knife and plunged it into her lover about 15 times, a court has heard.
Tabalbag, 31, admits to killing chef Amin Sthapit after finding him naked with Geecy Rebucas in the long-term couple's Darling Point apartment in November 2013.
But he says he is not guilty of murder because witnessing the affair provoked him to the point that his normal reasoning was suspended.
A NSW Supreme Court has heard Tabalbag saw the pair having sex through a partially opened front door and unlatched the security chain while Mr Sthapit hid in a bathroom.
He then forced his way into the bathroom and began stabbing Mr Sthapit before later following him around the bed in the small flat and stabbing him again, the jury was told.
During her somtimes grisly testimony, Ms Rebucas described Sthapit saying "hey, hey" throughout the fatal attack.
She said she told Tabalbag something like "please don't do anything, it's not his fault" after he entered the unit.
During the attack she saw Tabalbag stabbing Sthapit in the back as he leant injured on the bed, the court heard on Wednesday.
"I just can't believe he is capable of doing something like that," said Ms Rebucas, who moved from the Philippines with Tabalbag.
"He just keeps on saying 'why, why have you done this?".
Ms Rebucas says she tried on numerous occasions to get the knife or pull Tabalbag away from Sthapit and was hit in the head during the struggle.
Crown prosecutor Braddon Hughes SC said Tabalbag, who had downloaded software to retrieve deleted messages the day before the death, and used another app to find out Ms Rebucas was not where she said she was, planned the killing.
He told the court Tabalbag had said something like "look, this is what happens" after making his way into the unit.
But Tabalbag's barrister Belinda Rigg SC said some evidence suggested the attack was more "frenzied" than premeditated.
She expected the jury will hear he "is a completely non-violent and non-aggressive person".
"You'll hear this was nothing other than a completely loving, respectful and non-violent relationship," she said.
Ms Rebucas, who allegedly told Tabalbag to "run" after the stabbings, said she had never seen her former partner be violent.
The court heard Ms Rebucas met Mr Sthapit, a head chef, while they both worked at the Golden Sheaf hotel at Double Bay.
He gave her the chef's knife used in the killing, the court heard.
The trial continues.