Percy had his orders, and he was more than happy to carry them out. The Malfoys got away with far too much, and now that their little coup toppled the regime that had been so good to him? He was more than happy to be there, polyjuiced as Hermione, ready to cut down the traitorous woman. He wasn't really certain when his loyalties changed; Minister Thicknesse had made a lot of sense about the dangers of the muggle-born and their social ills. Squibs were being being born at a higher rate than ever, and morality was being undermined with this "anything-goes" mentality that had Ogden and Umbridge willing to dilute the Purebloods with the dirty, diseased blood of muggles.