Dumbledore tells Harry that Voldemort has used Dark Magic to save his own 'soul' at the cost of Myrtle's life. So Harry knows of at least one Dark Ritual which needs a death as sacrifice, ie making a Horcrux.
Quirrell tells Harry that the darkest ritual he knows requires only a sword that has slain a woman, and a rope that has hanged a man - to summon death.
Since Harry knows Quirrell knows every Dark Ritual there is worth knowing, and one of the rules of Evil Overlords is to get immortality, Harry knows that Quirrell is not telling him about the Horcrux ritual. If he puts the 'where to hide a Horcrux' & 'the lose 5 things' bits together, he will know he can never kill QuirrellMort, since he's unlikely to get to any of the Horcruxes anyway.
I don't think making a Horcrux qualifies as a ritual. And if it did, it requires one death whereas the one Quirrell mentions requires two (albeit indirectly and with the possibility of them not being murders).
He probably is hiding some darker rituals, but I doubt the making of Horcruxes is one of them.
(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: