Actually, her father says "how can I murder my own daughter? I will have to live with it". That's a close paraphrase.
What I meant by that was, the standard was set by the Supreme Court in ruling against the Cruzans (IIRC) in their first suit to kill Nancy. The court said they hadn't presented clear and convincing evidence that she would be better off dead, so that became the standard. "Clear and convincing."
>> "...I will have to live with it"
Her father evidently couldn't live with it very long. He killed himself -- in 1996, I think, about five years after Nancy's death.