Hermione was infuriated with Malfoy after he had used an Unforgivable on her, and she wanted vengeance for what he had done. She was going to even the scores a little. If Malfoy wanted to play dirty, she'd join in, after all, she was filthy herself. That's all he ever said about her, how revoltingly nasty she was. She'd show him just how disgusting she could be!