Motive: Michaela tells Rebecca point blank that her life is ruined and it's all Rebecca's fault. Micheala has lost her wedding ring, her future husband, and so much more. It would make sense that she'd want to get even.
Evidence: She's easily the most aggressive of the bunch when it comes to Rebecca. She's the one who wants to track her down. She's the one who backhands Rebecca and who threatens to destroy her. Perhaps, in the end, she keeps to her word.