When I first read the end of the chapter, my thought was that Quirrell hadn't arranged the incident; he had thought it was a "surprisingly good day" which suggested to me that he hadn't expected the troll.
After reading comments, I became less sure about that; someone suggested that Quirrell might have simply not intended for Harry to be at the scene and in danger. This seems plausible, but one thing still makes it difficult for me to believe it was Quirrell.
The troll had been enchanted against sunlight:
someone had enchanted the troll against sunlight before using it as a murder weapon and might also have strengthened it in other ways.
And Harry transfigured part of the troll:
Harry visualized a one-millimeter-wide cross-section through the enemy's brain, and Transfigured it into sulfuric acid.
But before, it was stated that Quirrell could not charm something that Harry had Transfigured:
Professor Quirrell could not cast spells on something Harry had Transfigured, for that would be an interaction, however slight, between their magics, but -
For Quirrell to have been behind this, I can see only two possibilities: 1) Harry can transfigure something Professor Quirrell has cast spells on, even though Professor Quirrell can't cast spells on something Harry has transfigured, or 2) Quirrell had a confederate (or imperiused lackey) cast the appropriate spells.
If you go with option 2, the increased complexity makes it less likely that it happened (though not necessarily less likely than a given alternative).
Oh snap. I didn't even notice that problem.
There is a third possibility: Dumbledore brought in the troll to guard the Stone (or other object), as in canon, and he was the one who cast the spells to protect it. I'm unsure about this, because it seems unusually violent for Dumbledore.
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: