Ruddelle, then 57, was initially charged with assault with a weapon.
But that charge was later withdrawn and replaced with one count of murder.
Ruddelle does not dispute that she stabbed Ngapera - but has pleaded not guilty to the murder charge.
She claims she was merely trying to defend herself and her "baby" from Ngapera's violent temper.
The trial, before Justice Matthew Palmer and a jury of eight women and four men, began this morning and is expected to take about two weeks.
After the jury was empanelled, Crown prosecutor Chris Howard opened the case against Ruddelle.
He said the night before the murder, Ruddelle and Ngapera went to the Crates and Cues bar in Manurewa.
They were "socialising and drinking, largely without incident".
"It was a good night, a good time," said Howard, adding that the couple were drinking steadily throughout the night.
At 3am they were dropped off at Ruddelle's address by her nephew.
"At that stage there was no sign anything was wrong, however about an hour and a half after they were dropped off emergency services received a 111 call from Ms Ruddelle's son," Howard said.
Ruddelle's son Potu Cameron was awoken by his 14-year-old brother.
Cameron found Ngapera lying on the floor of the dining room on his back - unresponsive, with no pulse and obvious stab wounds.
He called 111 and tried to revive Ngapera, but his wounds were fatal.
As he worked on Ngapera, Ruddelle was "hysterical".
"It is the Crown case that Ms Ruddelle stabbed Mr Ngapera in the chest, twice with a knife
with a 19cm blade," said Howard.
He said one of the blows penetrated Ngapera's heart.
Howard told the jury that Ruddelle and Ngapera had a rocky relationship.
At one stage Ruddelle had a protection order in place and Ngapera chased her around "threatening to bottle her".
He was charged and convicted in relation to that.
There were other domestic-related incidents during their time together.
But in September 2018, Howard said, the protection order was discharged at Ruddelle's request.
He said there "were no signs of animosity between the two" in the hours leading up to the death.
When police arrived at the scene they found a knife hidden behind a laundry basket in the lounge.
It would later be confirmed during the post mortem examination that Ngapera was stabbed once, then the knife was removed completely from his chest before the second blow.
Ruddelle was said to be extremely upset when police arrived at the scene - and made a number of comments about the alleged murder.
First, she was overheard saying "I just murdered somebody, someone I loved so much".
She then asked an officer "is Joseph gone?" and when told Ngapera was dead, Ruddelle was allegedly overheard saying "thank you, Jesus".
While speaking to another officer, Ruddelle started talking about the incident.
She told him that Ngapera "came at her like he always does", that she "grabbed the knife" to protect herself.
She signed the notes made by that officer, but then changed her mind and crossed out
her name.
Howard said that the jury may hear evidence that Ruddelle had been the victim of domestic violence - but they had to consider the murder charge in relation to the night of Ngapera's death.
He said there "might be" an issue of self defence during the trial.
"If she was, that would be a complete defence to the charge, and you will need to find her not guilty," he told the jury.
But they would have to be sure that Ruddelle did not intend to fatally wound Ngapera when she grabbed the deadly weapon.
"She could have used non-lethal force," he said.
"She could have left the room ... she could have called police.
"There was no reason to grab the knife."
The jury will start to hear from witnesses today including Cameron and other family members, first responders who attended the scene and people at the bar.
They will not hear from Ruddelle's youngest son as his family had not given permission for him to be interviewed.
Defence lawyer Shane Cassidy also addressed the jury today with an opening statement.
He said he wanted to alert them to the issues the defence would be asking them to consider.
"Karen did something that she had rarely - if ever - done before," he said.
"She called out to one of her own children for help.
"She and Joseph had been arguing and during the course of that argument he became angry and he came at her like he always does.
"But she knew what was coming."
Cassidy said two of Ruddelle's children were home that night - her youngest and Potu, an
adult.
"It was Potu that she called to for help ... but it was the 14-year-old who answered that call.
"At the age of 14, then stood between his mother and Mr Ngapera and shoved, pushed Mr Ngapera in the chest with both of his hands, forcing him against the wall.
"At that point in time Karen instinctively picked up what was near, a knife on the table, put herself between her child and her partner and stabbed Joseph twice in the chest to protect [her son] from what she believed was going to follow.
"But for the fact that [the boy] intervened when he did and in the way that he did - that particular use of force at that very moment would not have been necessary.
"It's that split second, that moment of time, that will be important to you in your deliberations."
Cassidy said it was the defence's case that Ruddelle loved Ngapera and wanted to marry him - despite his violence.
"She killed the man that she loved - but she did not want or intend for him to die," said
Cassidy.
"What she was thinking about, what was on her mind when she caused his death was her son.
"The defence says to you in those circumstances, given what she knew of Joseph, that she was entitled to protect herself and perhaps more importantly, her son."
The trial continues.
DO YOU NEED HELP?
If you're in danger now:
• Phone the police on 111 or ask neighbours of friends to ring for you.
• Run outside and head for where there are other people.
• Scream for help so that your neighbours can hear you.
• Take the children with you.
• Don't stop to get anything else.
• If you are being abused, remember it's not your fault. Violence is never okay.
Where to go for help or more information:
• Shine, free national helpline 9am- 11pm every day - 0508 744 633 www.2shine.org.nz
• Women's Refuge: Free national crisis line operates 24/7 - 0800 refuge or 0800 733 843 www.womensrefuge.org.nz
• Shakti: Providing specialist cultural services for African, Asian and Middle Eastern women and their children. Crisis line 24/7 0800 742 584
• It's Not Ok: Information line 0800 456 450 www.areyouok.org.nz
