It was as difficult to stop his relentless pacing as to refrain from shouting. Harry paused, biting his lip, and looked into Dumbledore’s lined face.
“Professor Snape made a terrible—”
“Don’t tell me it was a mistake, sir, he was listening at the door!”
“Please let me finish.” Dumbledore waited until Harry had nodded curtly, then went on. “Professor Snape made a terrible mistake. He was still in Lord Voldemort’s employ on the night he heard the first half of Professor Trelawney’s prophecy. Naturally, he hastened to tell his master what he had heard, for it concerned his master most deeply. But he did not know—he had no possible way of knowing—which boy Voldemort would hunt from then onwards, or that the parents he would destroy in his murderous quest were people that Professor Snape knew, that they were your mother and father—”
Harry let out a yell of mirthless laughter.
“He hated my dad like he hated Sirius! Haven’t you noticed, Professor, how the people Snape hates tend to end up dead?”
“You have no idea of the remorse Professor Snape felt when he realised how Lord Voldemort had interpreted the prophecy, Harry. I believe it to be the greatest regret of his life and the reason that he returned—”
“But he’s a very good Occlumens, isn’t he, sir?” said Harry, whose voice was shaking with the effort of keeping it steady. “And isn’t Voldemort convinced that Snape’s on his side, even now? Professor… how can you be sure Snape’s on our side?”
Dumbledore did not speak for a moment; he looked as though he was trying to make up his mind about something. At last he said, “I am sure. I trust Severus Snape completely.”