It was the confidence.

He had not treated her trust as something precious and fragile.

He had treated it as a weakness — a lever, a convenience, a thing to be used.

He had looked at the most generous thing she had ever given anyone and seen, only, an opportunity.

She had known, intellectually, that he was capable of dishonesty; the audit had told her that months ago.

But there is a difference between knowing a fact and hearing a voice, and the recording did something the documents could not.