duality
Emma wonders what the darkest side of her, now, would be. If she were to use that device to parse out all the facets that make up the darkness within her, what form would it take? She already killed her darkness once (and she never wants to think about that moment again, in the clearing, Killian falling to the ground.)
Killian tells her not to worry about these things, not with an evil queen on the loose. Not with Hyde hiding somewhere.
(She wants to laugh. Hyde-hiding. But laughter comes less easily these days.)