Harry Potter and the Methods of Rationality

"I'm not finished, son. The second reason is that the Killing Curse doesn't just take a powerful bit of magic. You've got to mean it. You've got to want someone dead, and not for the greater good, either. Killing Grice didn't bring back Blair Roche, or Nathan Rehfuss, or David Capito. It wasn't for justice, or to stop him doing it again. I wanted him dead. You understand now, lad? You don't have to be a Dark Wizard to use that spell - but you can't be Albus Dumbledore, either. And if you're arrested for killing with it, there's no possible defense."

"I... see," murmured the Boy-Who-Lived. You can't want the person dead as an instrumental value on the way to some positive future consequence, you can't cast it if you believe it's a necessary evil, you have to actually want them dead for the sake of being dead, as a terminal value in your utility function. "A magically embodied preference for death over life, striking within the plane of pure life force... that does sound like a difficult spell to block."

"Not difficult," Moody snapped. "Impossible."

Harry nodded gravely. "But David Monroe - or whoever - used the Killing Curse against a couple of Death Eaters even before they wiped out his family. Does that mean he already had to hate them? Like, the martial arts story was probably true?"

Moody shook his head slightly. "One of the dark truths of the Killing Curse, son, is that once you've cast it the first time, it doesn't take much hate to do it again."

"It damages the mind?"

Again Moody shook his head. "No. It's the killing that does that. Murder tears the soul - but that's just the same if it's a Cutting Hex. The Killing Curse doesn't crack your soul. It just takes a cracked soul to cast." If there was a sad expression on the scarred face, it could not be read. "But that doesn't tell us much about Monroe. The ones like Dumbledore who'll never be able to cast the Curse all their lives, because they never crack no matter what - they're the rare ones, very rare. It only takes a little cracking."

There was a strange heavy feeling in Harry's chest. He'd wondered what exactly it had meant, that Lily Potter had tried to cast the Killing Curse at Lord Voldemort with her last breath. But surely it was forgiveable, it was right and proper for a mother to hate the Dark Wizard who was coming to kill her baby, mocking her for how she couldn't stop him. There was something wrong with you as a parent if you couldn't cast Avada Kedavra, in that situation. And no other spell could've gone past the Dark Lord's shields; you'd have to at least try to hate the Dark Lord enough to want him dead for the sake of dead, if that was the only way to save your baby.

It only takes a little cracking...

"Enough," said Professor McGonagall. "What would you have us do?"

Moody's smile twisted. "Get rid of the Defense Professor and see if all your troubles mysteriously clear up. Bet you a Galleon they do."

Professor McGonagall looked like she was in pain. "Alastor - but - will you teach the classes, if -"

"Ha!" said Moody. "If I ever say yes to that question, check me for Polyjuice, because it's not me."

"I'll test it experimentally," Harry said. And then, as everyone looked at him, "I'll ask Professor Quirrell a question that the real David Monroe would know - like who else was in the Slytherin class of 1945, or something like that - hopefully without making it obvious. It won't be definitive proof, he could've studied the role, but it would be evidence. Still, Mr. Moody, even if Professor Quirrell isn't the original Monroe, I'm not sure that getting rid of him is a free action. He saved my life twice -"

"What?" demanded Moody. "When? How?"

"Once when he knocked down a bunch of witches who were summoning me toward the ground, once when he figured out that the Dementor was draining me through my wand. And if Professor Quirrell wasn't the one who set up Draco Malfoy in the first place, then he saved Draco Malfoy's life, and things would be a lot worse if he hadn't. If the Defense Professor isn't behind it all - he's not someone we can afford to just get rid of."

Professor McGonagall nodded firmly.

Hypothesis: Severus Snape

(April 8th, 1992, 9:03pm)

Harry and Professor McGonagall now stood on the slowly turning stairs, turning without descending; or at least one Harry stood upon those stairs - his other three selves had been left behind in the Headmaster's office.

"Can I ask you a private question?" Harry said, when he thought they were far enough away not to be heard. "And in particular, private from the Headmaster."

"Yes," Professor McGonagall said, not quite sighing. "Though I hope you realize that I cannot do anything which conflicts with my duties to -"

"Yes," Harry said, "that's exactly what I need to ask you about. In front of the Wizengamot, when Lucius Malfoy was saying that Hermione was no part of House Potter and that he wouldn't take the money, you told Hermione how to swear that oath. I want to know, if something like that comes up again, if your first duty is to the Hogwarts student Hermione Granger, or to the head of the Order of the Phoenix, Albus Dumbledore."

Professor McGonagall looked like someone had hit her in the face with a cast-iron frying-pan, a few minutes earlier, and now she'd been told that somebody was about to do it again, and not to flinch.

Harry flinched a little himself. Somewhere along the line he needed to pick up the knack of not phrasing things to hit as hard as he possibly could.

The walls rotated around them, behind them, and somehow, they descended.

"Oh, Mr. Potter," Professor McGonagall said with a low exhalation. "I... wish you wouldn't ask me such questions... oh, Harry, I wasn't thinking then, not at all. I only saw a chance to help Miss Granger and... I was Sorted into Gryffindor, after all."

"You've got a chance to think now," Harry said. It was all coming out wrong, but he had to say it anyway, because - "I'm not asking you to be loyal to me. But if you do know - if you are sure - what you'll do if it comes down to an innocent Hogwarts student versus the Order of the Phoenix a second time..."