ETA: There is now a third thread, so send new comments there.
Since the first thread has exceeded 500 comments, it seems time for a new one, with Eliezer's just-posted Chapter 33 & 34 to kick things off.
From previous post:
Spoiler Warning: this thread contains unrot13'd spoilers for Harry Potter and the Methods of Rationality up to the current chapter and for the original Harry Potter series. Please continue to use rot13 for spoilers to other works of fiction, or if you have insider knowledge of future chapters of Harry Potter and the Methods of Rationality.
A suggestion: mention at the top of your comment which chapter you're commenting on, or what chapter you're up to, so that people can understand the context of your comment even after more chapters have been posted. This can also help people avoid reading spoilers for a new chapter before they realize that there is a new chapter.
While in most types of fights killing is usually easier and safer than disabling, it is neither ruthless nor practical but simply short-sighted to lose track of why you are fighting.
There are plenty of situations where killing your enemy bears great costs. Maybe it would expose you to revenge from someone more powerful; maybe it would lead you to waste years of your life in court if not prison. Maybe it would be a massive PR victory for those who oppose you, undermining the entire plan that brought you into a fight in the first place (sounds familiar?). Maybe it would prevent you from obtaining critical help or information from the defeated enemy; the list could go on.
Of course preserving your own life has tremendous value (I heard there's a saying among some policemen: "an ugly trial is better than a beautiful funeral"), but it is not an absolutely incommensurable value. For a certain subset of X and Y in ]0,1[², you would accept an X chance of losing your life to a not-quite-dead enemy in exchange for a Y chance of not wasting decades of life (arguably), or (certainly) of not dying later on to unstoppable revenge.
All of the above being pretty much a very long-winded way of rephrasing Prof. Quirrell's dismissal that "there is a time and place to take your enemy alive, and a Hogwarts classroom is usually one of those".
I never got the impression that Harry's "Intent To Kill" thing included any tendency whatsoever to forget why he was fighting. The lesson here is completely distinct from being gratuitously stupid.