(The HPMOR discussion thread after this one is here.)
The previous thread is over the 500-comment threshold, so let's start a new Harry Potter and the Methods of Rationality discussion thread. This is the place to discuss Eliezer Yudkowsky's Harry Potter fanfic and anything related to it. The latest chapter as of 09/09/2011 is Ch. 77.
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: one, two, three, four, five, six, seven, eight. The fanfiction.net author page is the central location for information about updates and links to HPMOR-related goodies, and AdeleneDawner has kept an archive of Author's Notes.
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:
You do not need to rot13 anything about HP:MoR or the original Harry Potter series unless you are posting insider information from Eliezer Yudkowsky which is not supposed to be publicly available (which includes public statements by Eliezer that have been retracted).
If there is evidence for X in MOR and/or canon then it's fine to post about X without rot13, even if you also have heard privately from Eliezer that X is true. But you should not post that "Eliezer said X is true" unless you use rot13.
Actually, I just had a chilling realization in regards to that. From chapter 62:
'"No," said the old wizard's voice. "I do not think so. The Death Eaters learned, toward the end of the war, not to attack the Order's families. And if Voldemort is now acting without his former companions, he still knows that it is I who make the decisions for now, and he knows that I would give him nothing for any threat to your family. I have taught him that I do not give in to blackmail, and so he will not try."
Harry turned back then, and saw a coldness on the old wizard's face to match the shift in his voice, Dumbledore's blue eyes grown hard as steel behind the glasses, it didn't match the person but it matched the formal black robes.'
I strongly suspect that Dumbledore burned Narcissa Malfoy so that the death eaters would stop targeting the families of Order members. Judging by his tone of voice and body language in this excerpt, this is probably the one action during the war that Dumbledore most regrets having had to do.
If I'm right, Harry will be in a difficult moral situation when he learns the truth. Was what Dumbledore did justified? On the one hand, torturing a mostly innocent person to death is deplorable no matter how you slice it. On the other, if that was the only way to stop many other innocents from being tortured to death...
The theory I've been going with for a long time is that Dumbledore killed Narcissa accidentally while disposing of Tom Riddle's diary - he thought the house was empty and used Fiendfyre - and then later rationalized her death for the reasons you gave above.