You are trying to better understand what motivated someone to behave deceptively. They have already admitted to you that they lied. How should you approach the situation to minimise a defensive response?

Tell them all of the facts you already know about the situation, so they understand you will not accept any more of their lies

Ask them directly why they behaved the way they did.

Ask them what made them do what they did.

Be as silent as possible and just let them talk.