Voldemort is the only person in the world with an obvious motive for wanting to break Bellatrix out of Azkaban, and is who everyone else thinks is responsible
What motive would Harry expect Voldemort to have? As far as I can recall, he doesn't know about the components required for the spell to revive someone kept from death by horcruxes, and Bellatrix is not a very capable servant for the time being, and he doesn't believe Voldemort cared about her in any case. Quirrell, on the other hand, has already claimed a selfish motive that he personally has for freeing Bellatrix that would not apply to Voldemort.
Keep in mind that for Harry, the potential hypothesis space is huge. Quirrell might secretly be Rudolph Wizencamp in disguise. Don't know who Rudolph Wizencamp is? Well, neither does Harry, he's only lived in the wizarding world for a few months after all. We can reason by dramatic convention and conservation of detail, but for Harry, the list of all possibilities raised by the facts about the wizarding world that he's aware of is far from exhaustive.
Dumbledore told Harry in the "Today your war has begun" speech that Bellatrix was one of three things Voldemort needed to return as strong as he was before.
The next discussion thread is here.
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 85. The previous thread has long passed 500 comments. Comment in the 15th thread until you read chapter 85.
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.
As a reminder, it’s often useful to start your comment by indicating which chapter you are commenting on.
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: