I actually expected Harry to cast the Killing Curse as a last ditch desperation/rage effort. He knew what it does, has seen the wand movements and pronounciation (in the Dementor dream), knew and had the required state of mind. That should be enough to cast it, as per Ch26 ("He is in his sixth year at Hogwarts and he cast a high-level Dark curse without knowing what it did.").
I've used it myself. All it takes is power and a certain mood.
Harry may have had the mood, but there's doubt about the Power, and there's also been multiple foreshadows of how broken low-level spells are, and a recent mention that he's he can't stop himself from noticing them. Hence "censors off".
This is a new thread to discuss Eliezer Yudkowsky’s Harry Potter and the Methods of Rationality and anything related to it. This thread is intended for discussing chapter 88-89. The previous thread has passed 500 comments.
There is now a site dedicated to the story at hpmor.com, which is now the place to go to find the authors notes and all sorts of other goodies. AdeleneDawner has kept an archive of Author’s Notes. (This goes up to the notes for chapter 76, and is now not updating. The authors notes from chapter 77 onwards are on hpmor.com.)
The first 5 discussion threads are on the main page under the harry_potter tag. Threads 6 and on (including this one) are in the discussion section using its separate tag system. Also: 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18.
Spoiler Warning: this thread is full of spoilers. With few exceptions, spoilers for MOR and canon are fair game to post, without warning or rot13. More specifically: