"Be careful in fighting monsters lest you become one," Draco said.  He could tell by the blank expression on Potter's face he didn't recognize the reference or understand the implication.  Savior of the wizarding world and he was less well-read than Zabini.  It was sad, really.   Potter also didn't seem to realize he couldn't exactly make the Malfoy heir disappear without an investigation, and unless he wanted to keep Granger under lock and key as well, his family would know that where she was, he had to be. A brilliant strategist Potter wasn't.