I'm doing a reread.

"In any case, when I was thirteen years old, I read through the historical sections of the Hogwarts library, scrutinizing the lives and fates of past Dark Lords, and I made a list of all the mistakes that I would never make when I was a Dark Lord."

Harry giggled before he could stop himself.

"Yes, Mr. Potter, very amusing. So, Mr. Potter, can you guess what was the very first item on that list?"

Great. "Um... never use a complicated way of dealing with an enemy when you can just Abracadabra them?"

"The term, Mr. Potter, is Avada Kedavra," Professor Quirrell's voice sounded a bit sharp for some reason, "and no, that was not on the list I made at age thirteen. Would you care to guess again?"

Why does Quirrell react this way? I see two major possibilities.

1. He is picky about using the proper term. That's the surface appearance, and it jives well with the model of Professor Quirrell as a formerly evil teacher. But, we have to realize that ambiguous data might count as non ambiguous data when dealing with Professor Quirrell, because his skills at deception are so great. I think it's convincing that he might respond sharply to Harry misusing a term like that, but we don't see him do anything else in the book like that. And, if he was primarily focused on teaching, shouldn't he have been happier that Harry clearly understood the lesson he was trying to hammer into them, rather than focusing on a technical mistake? I think it's pretty convincing, but not an accurate explanation.

2. He took it personally. Specifically, this might have happened because he had the opportunity to simply avada kedavra Harry, but then he didn't. I don't believe that it's ever been confirmed in HPMOR that Harry actually got hit by the killing curse. Using more meta clues, Harry's alternative to avada kedavra'ing your enemies is a suspiciously specific sort of scenario, but yet also a very obvious lesson. Voldemort would have been upset that Harry so easily recognized what his mistake was. I also think Harry's lighthearted tone would do a lot to provoke Voldemort. The combination of these things might have made Quirrell's mask slip.

I guess I don't really think there's any strong evidence for two, but it's just interesting to me how reading the book two times before this allows me to reinterpret Quirrell's behavior. I enjoy this.