The whole part of Eve's lore was that no one knew what she was. She wasn't...possibly human. Nor was she a vampire or anything. No one knew what she was and that was her charm. She was unknown. Just an expensive assassin that lurked in the shadows for her next prey while having fun at it.
By flat out saying, "Oh she's a demon now." kinda spoils a lot of the mystery behind her character. I liked that the whole idea of using the word 'demon' was more of a symbolism to their character, Tahm being called demon didn't exactly mean he was one, but it was just something that could be called of him. "Call me King. Call me demon. Water forgets the names of the drowned."
By flat out turning her into one kinda just... I don't know. Rubs me the wrong way personally. I liked the ambiguity of her existence and her race.
I could probably be the only one that feels like this. So if I am, sorry. :P