Does the character think it’s wrong? Has she accepted being a monster in this way, or is she disgusted and in denial about it? Is this her big crime, or is she terrible in other ways? Because generally if she’s already kinda evil you won’t have to convince your audience that this is also evil, it’ll just be assumed
Societal clues, like how others perceive the attraction. Given, that’s a two sided blade, because not everyone’s perspectives should be taken to heart. Maybe try mixing the feeling of attraction in with other emotions. Basically, make the character question whether or not what she’s feeling is genuine attraction. I’ve never written something like this; sorry, if this isn’t super helpful.