As soon as she felt his grip loosen, she ripped her arm out of his grasp. She didn't notice what she had done until he heard her speak. Bellatrix raised his eyebrows at him. Yes, he was most certainly going to learn how to deal with pain.
An instinct was telling her to finish him off. A simple kick to the head would've done it. But she was supposed to be teaching him, not beating him to a bloody pulp. A moment later, she went to her bag and pulled out a towel. She wasn't going to coddle him. Bellatrix merely held out the towel for him to take.