I like the sound of that idea!! I, for one, would most definitely want to play/test this for you!
Dark games, especially dark RPG's, are a good thing seeing as to how many people are out there creating another cliché RPG in which you have to save the princess!!
I say: GO FOR IT!!!
Just to give you a heads on, when you do this, you have to consider into dragging the player into the character even more than in them cliché ones... Tell them everything they need to know about the character, give them inside information on their past and how they came to be 'evil minded', give them the thoughts of the character instead of just the conversation!