“And Dumbledore believed that?” said Lupin incredulously. “Dumbledore believed Snape was sorry James was dead? Snape hated James…”
“And he didn’t think my mother was worth a damn either,” said Harry, “because she was Muggle-born… ‘Mudblood,’ he called her…”
Nobody asked how Harry knew this. All of them seemed to be lost in horrified shock, trying to digest the monstrous truth of what had happened.
“This is all my fault,” said Professor McGonagall suddenly. She looked disoriented, twisting her wet handkerchief in her hands. “My fault. I sent Filius to fetch Snape tonight, I actually sent for him to come and help us! If I hadn’t alerted Snape to what was going on, he might never have joined forces with the Death Eaters. I don’t think he knew they were there before Filius told him, I don’t think he knew they were coming.”
“It isn’t your fault, Minerva,” said Lupin firmly. “We all wanted more help, we were glad to think Snape was on his way…”
“So when he arrived at the fight, he joined in on the Death Eaters’ side?” asked Harry, who wanted every detail of Snape’s duplicity and infamy, feverishly collecting more reasons to hate him, to swear vengeance.
“I don’t know exactly how it happened,” said Professor McGonagall distractedly. “It’s all so confusing… Dumbledore had told us that he would be leaving the school for a few hours and that we were to patrol the corridors just in case… Remus, Bill, and Nymphadora were to join us… and so we patrolled. All seemed quiet. Every secret passageway out of the school was covered. We knew nobody could fly in. There were powerful enchantments on every entrance into the castle. I still don’t know how the Death Eaters can possibly have entered…”
“I do,” said Harry, and he explained, briefly, about the pair of Vanishing Cabinets and the magical pathway they formed. “So they got in through the Room of Requirement.”