Quirrell and Voldemort are personas designed to play different roles. You are looking for different urges, but there are instead different purposes behind these roles, that call for different behaviors, with any urges controlled too reliably to manifest if contrary to the purpose.
Ch. 79 (Dumbledore):
But Voldemort was more Slytherin than Salazar, grasping at every opportunity.
Ch. 61 (Dumbledore):
It is too clever and too impossible, which was ever Voldemort's signature since the days he was known as Tom Riddle. Anyone who wished to forge that signature must needs be as cunning as Voldemort himself to do so.
Ch. 63 (Quirrell):
To an actor or spy or politician, the limit of his own diameter is the limit of who he can pretend to be, the limit of which face he may wear as a mask. But for such as you and I, anyone we can imagine, we can be, in reality and not pretense.
Quirrell is not Voldemort, Quirrell is Riddle, just as Voldemort is Riddle.
The simplest reason is that Quirrelmort is simply not in a position to indulge any sadistic impulses the way Voldemort was. He spends hours each day conked out completely, and he has no powerbase to retreat to. Overt malice of the kind Voldemort practiced would very rapidly earn him an adavra. There are quite a few other possible reasons - for one thing, Tom Riddle is not running on the same wetware anymore, and his original brain might have been miswired in a way that did not carry over, or heck, the original Quirrel could have been very calm and unflappable, so now Quirrelmort just cannot get a good temper tantrum going no matter how hard he tries.
The new thread, discussion 13, is here.
This is a new thread to discuss Eliezer Yudkowsky's Harry Potter and the Methods of Rationality and anything related to it. With three chapters recently the previous thread has very quickly reached 1000 comments. The latest chapter as of 25th March 2012 is Ch 80.
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, eleven.
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: