There seemed to be a fight going on inside Harry's mind between Hufflepuff One, who'd never trusted the Defense Professor in the first place; and Hufflepuff Two, who was far too loyal to Harry's friend, Professor Quirrell, to believe something like that just because Moody said so.
It is kind of obvious, though, observed his Slytherin part. I mean, do you actually believe that under natural circumstances, anyone would end up as the last heir to a Most Ancient House AND Lord Voldemort killed his family AND he has to avenge his martial arts sensei? If anything I'd say he went too far over the top in setting up his new identity as the ideal literary hero. That sort of thing doesn't happen in real life.
This from an orphan who was raised unaware of his heritage, commented Harry's Inner Critic. With a prophecy about him. You know, I don't think we've ever read a story about two equally destined heroes competing to see who's cliched enough to take down the villain -
Yes, replied the central Harry over the distant vroop-ing noise in the background, it's a very sad life we lead and YOU'RE NOT HELPING.
There's only one thing to do at this point, said Ravenclaw. And we all know what it is, so why argue?
But, Harry replied, how do we test experimentally whether or not Professor Quirrell is the original David Monroe? I mean, what sort of observable behaves differently, depending on whether he's the real David Monroe or an impostor?
"What do you want me to do about it, Mad-Eye?" Professor McGonagall was demanding. "I can't -"
"You can," the scarred man said, glaring at her fiercely. "Just fire the bloody Defense Professor."
"You say that every year," said Professor McGonagall.
"Yes, and I'm always right!"
"Constant vigilance or no, Alastor, the students must be taught!"
Moody snorted. "Pfah! I swear the curse gets worse every year, as you lot get more and more reluctant to let them go. Your precious Professor Quirrell would have to be Grindelwald in disguise, to get himself sent off!"
"Is he?" Harry couldn't help asking. "I mean, could he actually be -"
"I check Grindie's cell every two months," Moody said. "He was there in March."
"Could the person in the cell be a ringer?"
"I administer a blood test for his identity, son."
"Where do you keep the blood you use as a reference?"
"In a safe place." Something like a smile was stretching the scarred lips. "Have you considered the Auror Office after you graduate?"
"Alastor," Professor McGonagall said reluctantly. "The Defense Professor does have a... health condition. I suppose you will call it suspicious in itself - but it is by no means certain that it will be any ill-doing on his part which prevents us from renewing his employment."
"Yes, his little naptimes," Moody said darkly. "Amelia thinks he stepped into the path of a high-level curse. Sounds to me more like a Dark ritual gone wrong!"
"You've no proof of that!" Professor McGonagall said.
"That man might as well be wearing a sign saying 'Dark Wizard' in glowing green letters over his head."
"Ah..." Harry said. It didn't seem like an especially good time to ask what Mr. Moody thought of the 'not all sacrificial rituals are evil' standpoint. "Excuse me, but you said earlier that Professor Quirrell - I mean the old David Monroe - I mean the Monroe from the seventies - anyway, you said that person used the Killing Curse. What does that imply? Does somebody have to be a Dark Wizard to use it?"
Moody shook his head. "I've used it myself. All it takes is power and a certain mood." The grimacing lips were showing teeth. "The first time I cast it was against a wizard named Gerald Grice, and you can ask me what he did after you graduate Hogwarts."
"But why is it Unforgiveable, then?" Harry said. "I mean, a Cutting Hex can kill someone too. So why's it any better to use a Reducto instead of Avada Kedav-"
"Shut your mouth!" Moody said sharply. "Someone might take it the wrong way, your saying that incantation. You look too young to cast it, but there's such a thing as Polyjuice. And to answer your question, boy, there's two reasons why that spell's in the blackest book. The first is that the Killing Curse strikes directly at the soul, and it'll just keep going until it hits one. Straight through shields. Straight through walls. There's a reason why even Aurors fighting Death Eaters weren't allowed to use it before the Monroe Act."
"Ah," said Harry. "That does seem like an excellent reason to ban -"
"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.