Thanks, I think part of my surprise was from a different understanding of "he'll feel forced to kill Quirrel", to me that means "he'll take the decision of trying to kill Quirrel, using whatever indirect plan, surprise, henchmen, ... in the process", not just a one-to-one fight in which Harry kills Quirrel (like the way he kills Voldemort in the cannon). I agree the probability of such one-to-one fight is quite low.