This occurred to me, but Hermione really shot herself in the foot by publicly demonstrating the ability to successfully cast a charm above her year level on the first try, from nothing more than having glanced at the instructions once.
Plus, just from naming conventions I would expect that spell to have non-lethal uses- it's not called the Blood-Cooling Curse, or even Hex, but Charm. Maybe it's a treatment for heatstroke, or the counter to the Blood-Boiling Curse. In any situation other than under Hogwarts' wards, a Charm that takes six hours to kill an unconscious first-year isn't that big a deal, compared to, say, Levitating them off a cliff and dropping them.
True, but it's a question that might get asked under Veritaserum.
(That said, we are talking about Hermione here. She can probably learn the spell from the false memory of having cast it once. So this test isn't likely to be conclusive.)
EDIT: New discussion thread here.
This is a new thread to discuss Eliezer Yudkowsky's Harry Potter and the Methods of Rationality and anything related to it. With two chapters recently the previous thread has very quickly reached 500 comments. The latest chapter as of 17th March 2012 is Ch. 79.
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: one, two, three, four, five, six, seven, eight, nine, ten.
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: