Shoelip -> Obviously, you underestimate the "bad guy" fantasy. Even if Prodigal was a man, it wouldn't have change nothing (except a screaming girl fan base. Just kidding, especially as I am a girl too ; ) It's all about feeling "special", so special that even the bad guy/girl would want you, would want to go to such extend just to have you.
Of course, it's a little disturbing, especially the more questionable the bad guy/girl is. I personally think it's a question of balance between redeemable and questionable features.
Also, my apologies if I didn't develop my point of view about my disagreement. I didn't want to make a novel but I will try to answer you ^^
First of all, if Prodigal was a poorly written character, there wouldn't be so much debate around her character. Yes, in The Prodigy, she's a psycho, a villain but as she said herself: a hero can't exist if there isn't a villain to face. Think about the Joker for example, he's also a psycho who had a tragic past and was driven insane because of it.
Batman had a similar tragic past but he chooses the path of justice instead of crime, even if there's proof that he's himself a little crazy because he dresses himself as a bat man. It's all about choice. The same thing was underlined in Harry Potter when Harry asked Dumbledore if he was the same as Voldemort because they share similar features. Dumbledore answered that Harry didn't choose the same path as him.
Regarding Prodigal, she was always alone. The MC had her grandma after the arrest of her parents but no one was here for Prodigal. No one to guide her, to shelter her, to teach her about responsibilities regarding her powers. Prodigal is like everyone: everyone need anchors, references, goals to give life a meaning.
For some people, parents and relatives are the obvious choice to serve as reference for a child, whether one would want to follow the path of her parents or rebel against it. For Prodigal, the only one reference she had was the path of her mother while you had your schoolmates, grandma, Jenny, etc... Without forgetting the fact that your parents were still alive while she believed her own mother dead.
But her mother path was not enough to Prodigal, she needed another anchor to her life, someone to identify with. She decided to choose you, the child of her mother arch-enemies. It can be interpreted in three ways: as a way to forget her loneliness, as a way to have the impression of having a relative (you) and as a way to give her life a meaning.
She refused to be another anonymous in the crowd. The burdening of her mother Legend was too much for her to handle by herself. When you take the time to think about it, she's a broken, vulnerable character with a weak will behind the villain mask.
So yes, in the end, if you can understand that, you can't really hate her so your point of view is accurate on this point. A philosopher said that when you understand something, you can't hate it. Hate comes from fear, and fear comes from the unknown.
As you can see, I can speak volumes about the Prodigal character, a simple proof that if we can explain her reasons and behavior, that must mean that the author was extremely thoughtful when he wrote the character.
She was poorly written in HeroFall only in her romance route, because her behaviour and the MC behavior were offbeat in regard of their common past and personalities.