Hermione did as she had been ordered by her Lord. She was polite, friendly, and kind to every mortal in her house. She had a part to play, and these mortals would respond better to the mask she wore. She was a model student, and a model citizen. She gave the Dark Lord an air of legitimacy with those who had previously questioned whether His reputation at school was a front. After all, if His betrothed was also kind then surely His politeness was true.