Why I think he's doing this :
so Draco doesn't know that Harry probably killed his father ; Harry values his friendship with Draco and doesn't want to lose it ;
so Harry doesn't have to tell everyone about his secrets (like partial transfiguration) ;
so they don't search for Harry for the transfigured Voldemort ;
so Harry doesn't have too many legal/political problems for actually killing dozens of people, including some very powerful ones ;
to give some credit and status to Hermione, which at this point Harry trusts more than himself to take the ethical decision (and not destroy the world) ;
to save the image of Quirrel, I think Harry still has emotional commitment to the character of Quirrel, even if it was just a mask, and doesn't want that image to be destroyed.
Preserving the image of Quirrell also helps in continuing to restore Slytherin, whereas outing him could damn the house to be forever ignoble or be removed completely
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 116.
There is 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.)