Reasons! Plot! Or maybe because Hero! Rumple will win and snuff (twitch) out the darkness which has been Emma’s endgame all along. They basically gave it away this past week when she said something about using all that power for good.
I thought the point of that dialog was that the darkness won’t be destroyed in the end, but rather someone will be worthy enough to wield the power without turning evil and use it to protect people. It’s Merlin who seems to want to go with the easy solution of just destroying it altogether so they’ll “never have to ask that much of anyone”.
That is true, yes. But I meant that Emma embraced her powers in order to do something that is good. I don’t know that Emma is evil; she’s just using the power covertly in order to bring about the desired end. She has yet to do anything Dark One-ish in present day.