One reason Harry might not use the killing curse is that he wants to avoid the slippery slope that Mad-Eye warned of--once a wizard has used the curse once, it becomes easier to do so in the future.
Moody said the slippery slope was due to killing, not due to casting the curse, and Harry still killed the troll. Quote:
...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 [emphasis mine]. 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
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: