My sincere thanks, I missed that.
"That is your condition for helping me to obtain the Stone?" said Professor Quirrell. Harry nodded, unable to form words.
Does that mean that Voldemort is freed from his previous offer to resurrect Hermione? Voldemort specifically says "That is your condition" which Harry agreed to. If so, Harry is going to be kicking himself later.
I think Solipsist was trying to say that if Voldemort using Sprout to cast a spell counts as originating from Voldemort, then if Sprout (or some other Imperiused individual) cast a spell on the troll's skin, then Harry shouldn't have been able to touch the troll. Before, it was assumed that Harry was able to touch the enchanted troll (its skin had been enchanted to resist sunlight) because Voldemort hadn't directly enchanted it (Harry can't touch things Voldemort enchants, see the broomstick escape from Azkaban) but used an Imperiused confederate. Now, however, it seems that using the Imperiused confederate does not negate the issues arising between Harry and Voldemort's magic.
To reply to Shminux, I imagine it's because the sense of doom is some magical warning of the repercussions of Harry and Voldemort's magic colliding, which probably is not being set off by the probability of Harry's life being ended by a high-velocity piece of metal fired from a completely non-magical weapon.
To Alsadius, I think the gun is more deadly to Harry than an Avada Kedavra because of Harry's previous resistance to the spell. Remember when Quirrell previously suggested that they stage a "return of Voldemort scenario" where the pretend-Voldemort shot another killing curse at Harry who would block it with his Patronus 2.0? Harry's reaction was something along the lines of, "Nobody would believe that Voldemort would be so stupid to try that again." Apparently, Harry's ability to block it (as demonstrated in Azkaban) and previous resistance (as demonstrated the night his parents died) made Voldemort think the same thing.
Why would Dumbledore enchant the troll against sunlight if it was going to be in the third-floor corridor all along?
Good point, I missed the picking the troll up by the ear entirely.
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).
"I wonder how difficult it would be to just make a list of all the top blood purists and kill them.
They'd tried exactly that during the French Revolution, more or less - make a list of all the enemies of Progress and remove everything above the neck..." -Harry's internal monologue, HPMOR Chapter 7
"Amusing, but that was not your first fleeting thought before you substituted something safer, less damaging. No, what you remembered was how you considered lining up all the blood purists and guillotining them. And now you are telling yourself you were not serious, but you were. If you could do it this very moment and no one would ever know, you would." -The Sorting Hat, HPMOR Chapter 10
Well...well I guess it wasn't technically a guillotine. And Harry didn't make a list himself. But Harry did do it, and set it up so no one would ever know.