"The mood soured and things became heated," Justice Peters said.
Later the situation deteriorated further and at some point Anderson attacked Witehira's mother, the high court judge said.
"Most people would not stand by and watch their mother be strangled. "
The sight "would have been intolerable" to Witehira.
She opted for a fight rather than flight response, the court heard.
Witehira picked up what she said she believed was a pencil but in reality it was a knife.
Emergency services arrived but could not resuscitate Anderson.
Justice Peters said alcohol played a significant part in how people behaved that evening.
Anderson was the second eldest of four siblings, and enjoyed his work as a truck driver.
A statement on behalf of his father was one of four victim impact statements read to the court today.
"I deeply miss my son Colin and it is such a tragedy that he was taken away from me."
Witehira herself spoke tearfully to the court, saying she was deeply sorry for the pain and heartache she had caused.
"I can apologise a million times and still I know that will never be good enough," she said.
"I know there is nothing I can say that will ever bring him back.
"He did not deserve to die."
She hoped one day the family could forgive her and find peace.