It was clearly within Quirrelmort's power to subdue Bahry without escalating to the use of the killing curse. He wasn't even exerting himself during the duel; if he needed to position Bahry for some reason, he could simply have started to pursue and maneuver. Bahry was clearly already on the psychological defensive, and was being forced to dodge his attacks, so using the killing curse is redundant for those purposes. Bahry's own monologue notes that his magic was almost completely exhausted. He wouldn't have been able to hold out much longer anyway.
If Quirrelmort had been forced to maneuver Bahry out of the way of his own curse, it would show Bahry that he was not actually trying to kill him, which stands to lower Bahry's threat estimate of him and further galvanize his resistance, because Bahry will know Quirrelmort is committed to taking him alive. Bahry is certainly not going to decide to surrender because of Quirrelmort's willingness to kill him, since he's already been using potentially lethal spells, and he's already aware that he's outclassed. He's implicitly prepared to go down fighting. It's simply not clear how using the killing curse is useful in this situation.
When Quirrelmort used the killing curse, I noticed that I was confused, and after his explanation, I noticed that I was still confused. Quirrelmort's explanation simply doesn't add up.
- This thread has run its course. You will find newer threads in the discussion section.
Another discussion thread - the fourth - has reached the (arbitrary?) 500 comments threshold, so it's time for a new thread for Eliezer Yudkowsky's widely-praised Harry Potter fanfic.
Most of the paratext and fan-made resources are listed on Mr. LessWrong's author page. There is also AdeleneDawner's collection of most of the previously-published Author's Notes.
Older threads: one, two, three, four. By tag.
Newer threads are in the Discussion section, starting from Part 6.
Spoiler policy as suggested by Unnamed and approved by Eliezer, me, and at least three other upmodders:
It would also be quite sensible and welcome to continue the practice of declaring at the top of your post which chapters you are about to discuss, especially for newly-published ones, so that people who haven't yet seen them can stop reading in time.