The only retort Bellatrix intended to offer Millicent was the killing curse itself and the words were half formed on her lips when Edgar's disarming spell hit her and her wand flew out of her hand, skittering across the floor. Irritation at actually being hit with an expelliarmus of all things would come later - for now Bellatrix was moving on sheer instinct and she dove for her wand, narrowly avoiding Edgar's stunning spell in the process.
It was only once her wand was back in her hand and she had blindly sent a blasting curse towards the doorway that she pulled herself upright and looked to see just who her attacker was. Bones. Lovely. That meant Dumbledore's band of vigilantes had been alerted and where there was one, there were surely more to follow. She would just have to deal with her foolish cousin quickly then. Millicent could wait the few minutes it would take her to deal with Edgar, she decided as she sent a silent incendio flying at him.