That one action may have had a reason: to demonstrate to someone how loyal she was to him, perhaps, or what was possible with Dark rituals for mind control.
More generally, the public Voldemort persona - as exemplified by Harry pretending to be Voldemort to Bella - is very different from Quirrel's public persona. In my impression, Quirrel's is more personable and likeable, and just as powerful and scary and competent. This may be due to complexities of the Quirrel-Voldemort relationship.
I don't know, it's kind of telling that Harry-as-Voldemort quotes Quirrell's "one must be efficient" thing to Bella without seeming to notice. And I've always been of the opinion that the glowing-red-eyes, high-pitched-giggling schtick was a conscious attempt to get people to underestimate him ever since the martial arts story. (He goes in disguise to learn, is taught to "lose" and kick butt, comes back afterwards in full regalia to kill everyone who spat on him except his one friend, in the process making sure no one else will learn what he did.) Apparently he sees a benefit in cultivating the crazy sadistic killer persona, but it's definitely done out of cold calculation.
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: