Horcruxes or not, that thing of a creature could not exist with a corporeal form. Could not give commands. Could not cause hurt. So Quirrell was going to die. But Harry was tired of playing with his life, so he'd have to plot first. And to plot, they needed to converse. And to converse, he had to get this stupid conversation with McGonagall over with, even when the reason he was in that bloody forest to begin with was because of her! He thought he could trust her and she sends him to hunt dead unicorns in the forest with Hagrid at midnight?