The woman, 44, admits to stabbing her stepfather 21 times in the chest, abdomen, back and arm, at his house in Grimstad, Norway.
"I thought that now he will not be able to destroy more children," she explained to police. "He must be punished."
"Her own upbringing and her descriptions of the sexual abuse of her sister and children justify her actions," the woman's defence lawyer Svein Kjetil Stallemo, told the Aust-Agder District Court on Monday.
He said that the woman had never been able to reconcile herself to the fact that her stepfather had been acquitted of sexual assaults she claims he committed against her daughters a decade ago.
When one of her daughters called her from a mental institution, where she had been committed as a result of her alleged trauma, the woman snapped.
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As soon as she finished the call she said, she grabbed a kitchen knife with 20cm blade and drove by scooter to her stepfather's house five kilometres away, where she stabbed him to death in his bed.