He is carrying Lord Voldemort on the back of his head. As a result, he has to bow to Voldemort's wishes, whatever they may be, and is subject to whatever punishments Voldemort chooses to dish out. Quirrell is trying to attach himself to a source of power, and has found that the source of power has attached itself to him instead. The relationship between Quirrell and Lord Voldemort is rather tighter than any relationship should be. Quirrell has adopted a stammering, uncertain pose in order to remove suspicion from himself, and admits that it has been useful to have Snape fluttering around like a great black bat to attract all the suspicions onto himself. Snape and Quirrell do seem to share some goal, and to be fighting between themselves, during the course of the first book; they often seem to be arguing.