Ok I think this was the explanation I was thinking of. https://mubi.com/reviews/26849
TLDR: The film doesn’t EVER shows us the real Asami. Neither the submissive girl at the beginning, nor the psychotic fiend at the end is really her. All we are ever seeing is Aoyama’s own perceptions of her in each phase of the relationship; perceptions which are not real but instead are colored by his desires and guilt and then projected onto Asami.