The Dark Lord cared not for riddles. Nagini's response, therefore, simply angered him. Were she anyone else, the clearing would have glowed green at that moment. But she was someone special, someone who possessed something too valuable to risk.
"Is that to be your only answer? Am I now to suspect your every move because you refuse to tell me your motives"
"Trust me, my Lord," Nagini said cooly, "When it is needful, my motives will become apparent"
"Who's to be the judge of when it's 'needful"
"Must you even ask"

Draco was becoming steadily more angry with Snape. He had made it clear that he didn't want anything to do with his former Professor, and yet the man insisted on hovering around him, his eyes ever watchful. Draco felt that Snape was just waiting for him to screw up again. Then Snape could tell the Dark Lord how unfit he was, how he was a disgrace to the name of Malfoy. Draco already knew all of this, and he dreaded the moment when the Dark Lord found out.