Now that's officially some batshit crazy talk going on over there.
I don't see the big-bad here effectively withholding her emotions or failing to be swayed by them/those of others. She appears to be expressing them in a vain similar to Maleficent, the Dark Fairy (how the Faë courts were viewed to have acted in some fiction).
She put a nightmare curse on the MC. She is slowly torturing her to death (that exhibits a ton of emotion). If she challenged her, the MC fears that the Big-Bad would blow her away on the spot. The Big-Bad appears to effectively hide her emotions rather than acting on them. She get revenge slowly, methodically. Her emotions explode into long-thought-out means of making the MC suffer. Very antisocial type personality behavior.
Though, I would agree that the Big-Bad makes for an effective villains. She is better equipped to hide her emotions that when she flies into a blinding rage she may exact her revenge ruthlessly, slowly, and grotesquely to her "pawn."