Draco had been surprised by Potter's suggestion that Granger should be the one to destroy the Cup. He would have kept the damned thing as far away from her as possible. But Potter argued that Granger was as much a part of this as anyone else. And since Draco had destroyed the diadem, Potter had destroyed the locket in joined forces with Weasley the Wanker, it was only fair Granger got to destroy the Cup.