@Xhunn She couldn't very well kill him due to Karna. He's a goody two-shoes. She would risk losing him by slaying the useless hero, or probably any humans, so that he would be aware of it. Even the kid's love of money comes from wanting to help his grandparents. I believe he's far stronger than Cecilia, so he's a gift from heaven for her, as long as she can keep him charmed. He'll be a huge help in keeping other demon lords from threatening Cecilia too much. The current situation is splendid for her. Perhaps if she can make him fall for her for real, it would be even better. It might also serve her well if Karna himself never needed to kill humans as it would keep his personality more soft and naive, and thus easier for her to manipulate.