Years of experience tend to build up the threshold of pain a person finds tolerable, and while the impact on his chin is definitely nothing to be sneezed at all Sarge does is grunt and pull down harder, trying not to flinch at the rather sickening sound her fingers are making.
There is nothing he can do about the way his brain is wired and it still takes some effort to actively hurt her, but she does do everything she can to help him get past that. Normally he would try his best to avoid it but she is just a crazy Ghoul. Maybe that will help him justify trying to break her fingers and not minding it as much as he should.
"Where's your crazy talk now, huh?" Normally he would have let a few blows rain down on the back of her head and he knows that it won't help him win this fight of he holds back on her like that. So he pats her head instead, hoping to enrage her further so she can pull him with her past the point of caring.