She set me up to kill Chuck and Amara so she could become the new God and put everything back in the natural order. So everyone who was supposed to die would die, and our friends from the other universe would die too. She was using me as her weapon.

[ Which isn't new for Jack so he says it in a sort of world weary way, like it's expected at this point. He trusts that Castiel wouldn't, but he clearly has been by other people before. He did trust Billie, there was logic in her plan, and he never questioned what would happen afterward, and who would fill the God-sized hole left behind. Jack is very bad at asking questions or thinking past what he's told, he's still working on that. ]

She still wanted me, she took me back out of the Empty, but Dean stopped her. Her next conflict with Dean and you was what led you to be taken from us. Dean and I really believed in her, but Sam already suspected she was hiding things.