To add to this, what will be done with Bellatrix after she is freed? Wouldn't Harry need an answer to this before cooperating with Quirrell?
Simply releasing Bellatrix to her own recognizance would be like releasing a hungry lion near a grade school during lunch hour. Without Voldemort personally directing her actions she would act out of her own sense of vengeance.
It isn't obvious to me that a simple "trust me" from Quirrell would convince Harry to cooperate.
I thought this was weird too, but EY has a habit of revealing plans as they are enacted rather than beforehand.
[Update: and now there's a fifth discussion thread, which you should probably use in preference to this one. Later update: and a sixth -- in the discussion section, which is where these threads are living for now on. Also: tag for HP threads in the main section, and tag for HP threads in the discussion section.]
The third discussion thread is above 500 comments now, just like the others, so it's time for a new one. Predecessors: one, two, three. For anyone who's been on Mars and doesn't know what this is about: it's Eliezer's remarkable Harry Potter fanfic.
Spoiler warning and helpful suggestion (copied from those in the earlier threads):
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.