What does Gene do when Quackenbush accuses him of being maimed? What motivates Gene to respond this way? In what sense might Gene be maimed?

Geene reacts by hitting Quackenbush in the head. I think this doesn't have so much to do why Gene himself but rather the fact than Finny is maimed. Gene's deep-seated guilt over Finny's injury is brought to the surface.   Gene feels so much guilt that he feels maimed himself.