Because she was the bad guy... though if you ask me, she was just a pawn for the REAL villain, Amshel. At least that what it seemed like to me. Poor Diva was nothing more than a proverbial lab rat and she had no true concept of right and wrong, not like Saya did. All she ever wanted was love from a real family, though she went about claiming that the entirely wrong way.
Diva wasn't evil. She was just a victim.