The court was told that after returning home from a local bar the pair argued in Ruddelle's Manurewa home.
Ruddelle's 14-year-old son woke and jumped in between the pair to protect his mother.
She then picked up a 19cm knife and stabbed Ngapera twice, piercing his heart.
He died soon after despite efforts by Ruddelle's son to save his life.
Ruddelle does not dispute that she killed the 56-year-old - but denies it was murder.
She says her actions were in self defence - that she was terrified of Ngapera because of years of domestic violence and that he initially lunged at the knife.
The defence says Ruddelle suffers from battered women's syndrome - now known as social entrapment- and that Ngapera used coercive control over her.
Yesterday the court heard that Ruddelle called police 16 times during her relationship with Ngapera, to ask for help when he was being violent, aggressive, threatening or intimidating.
At one point the pair broke up and Ruddelle sought a protection order - but had it discharged two months before Ngapera died.
At the beginning of her evidence she spoke about growing up with alcoholic parents in a house where violence was part of daily life.
And she outlined her turbulent and dangerous previous relationships before she turned her mind to Ngapera.
The pair met in 2015 and he was homeless but Ruddelle saw "the good" in him and embarked on a relationship.
It did not take long for the violence to being.
The first time he assaulted her was after a night drinking at home with friends.
"It was early in our relationship ... I don't know if I said anything to provoke him, we were just having a conversation," said Ruddelle.
"All of a sudden he grabbed me by the throat and I was up against the wall and he was strangling me.
"He had me up against the cupboard wall with both of his hands ... I tried to say 'stop Joe this is not you this is not who you are'.
"He had me tight ... he let go and then he grabbed me again by the throat.
"I thought I was going to die."
Another time Ruddelle said Ngapera fell out of bed when he was hungover.
He stormed into the kitchen and accused her of pushing him.
"He said 'hey bitch, you pushed me off the bed' and then he punched me quickly in the head with his right hand, twice," she recalled.
"He just punched me so hard that I fell to the ground.
"I was dizzy and confused because I didn't do that ... I thought he was gonna kill me because I had witnessed him strangling me before that."
She explained that part of her fear of Ngapera came from him telling her he had murdered someone and served time in Australia, that he had beaten a homeless man to death in Auckland and that he bashed a taxi driver for ripping him off and "could have killed him".
The trial, before Justice Matthew Palmer and a jury, continues.
