I understand where the other demon is coming from. She knows Tanjiro's right. When she sees Nezuko, she realizes they might accept her. However, the brother is so strong she doesn't think anyone can beat him. She's in a lose-lose situation either the demon hunters kill her or he puts her through hell. It's all she knows, so she continues on in her hell. With luck Tanjiro will save her, but I don't think so. Sad.