I suppose Arthur is the true villain of 5A. I think he's only interested in taking the dagger to complete the sword, and will use his Hero cover to make believe his intention are pure.
Emma will I guess, somehow discover it. But beeing the dark one means she will be less reliable than she was as a savior, and Arthur will probably use this opportunity to discredit her. Snowing and regina are sometimes ungrateful, and Emma seeing them don't trust her after all she did to save everyone, will finally let the darkness consume her. This could explain her attitude to regina in the sneak peak for 5x02. She clearly want Regina to live wath she's been through. This is typical of vengeful and misunderstood people. The show is reinterpreting tales to recall our reality. And since saison 4 this is all about role switching, trust and hope. The villain beeing the hero, the hero beeing the villain. The idea of trusting in people we're supposed to love. It's like popular people pretending to preach goodness while unpopular and misunderstood ones, are condemned for heros mistakes. This is how half of antagonists are made in our world.
So wath do you think about that theory?