Board Thread:Spoilers!/@comment-5899967-20150912000118/@comment-24674051-20150915135715

Actually, I could see Zelena choosing to become an unlikely ally. Just because Emma is the dark one does not mean that she does not also have access to her light magic. And a Dark Emma just might do what Regina wouldn't and Rumplie failed to do, and that is kill her. After all, according to Glinda, only the strongest purveyor of light magic can defeat Zelena, and that would be Emma. So, with Emma Dark, Zelena's interests suddenly become aligned with everyone elses, even if it is for personally selfish reasons.