Well, there’s a lot of rabid love for Regina, who killed her own parents, and her husband, and tried multiple times to kill one of her (step)children…
Along with entire villages along the way. And random guards. And her boyfriend/sex slave.
If people can find her a sympathetic character (ditto Rumple), I should think that, given a good storyline and character depth, we could still love a truly villainous princess.
I’m not sure it could happen for a “family audience” though. Elsa may be the closest we can get if it’s at all deemed “for children.” (I would argue that Disney animation is NOT just for kids in any way, but since that is a major part of the audience that they have to consider, along with parents who will freak out over every little thing…it’s a relevant consideration.)
Keeper of Belle's Gold magic, sand dollar, cloaks, purple FTL outfit, spell scroll, library key, copy of Romeo and Juliet, and cry-muffling pillow, Rumple's doll, overcoat, and strength, and The Timeline. My spreadsheet: https://drive.google.com/file/d/0B6r8CySCCWd9R0RUNm4xR3RhMEU/view?usp=sharing