(don't do that with Gratiana's, she wants to absorb her soul herself, just hand it over and all quests done).
Gratiana character in the game is practically the same monster as Braccus Rex's, so anyone half decent morally casual gamer absolutely should kill her : smash her soul jar in front of her (this also gives more experience points). She's useless anyway and don't appears in the game after act 1.
