What does Gene do when Quackenbush accuses him of being maimed in A Separate Peace?

When Quackenbush accuses him of being maimed, Gene reacts by hitting him hard across the face.  At first Gene doesn't know why he is reacting like this, and he thinks, "it was almost as though (he, Gene) were maimed".  Then he remembers Finny, who is now truly maimed physically after falling from the tree, and believes that he is coming to his friend's defense.  Gene fights that "battle, that first skirmish of a long campaign, for Finny".  Yet, when the fight is over, "it didn't feel exactly as though (he) had done it for Phineas...it felt as though (he) had done it for (him)self".

On the surface, Gene reacts with such rage at Quackenbush's scorning of someone "maimed" to defend Finny, but in reality, Gene comes closer to the truth when he notes that his violent response has more to do with himself.  Tormented by his own insecurities and the petty jealousies he feels towards his "best" friend, he lives with a deep burden of guilt over the idea that he might have purposefully caused Finny's accident.  Gene despises himself, for his lack of resolve which results in him always being a follower where Finny is concerned, and for the horrible deed in which he finally lashes out at Finny, seriously hurting him.  Gene has taken the job of "assistant senior crew manager", a job "usually taken by boys with some physical disability", because it is a "nothing" job, and he wants to fulfill his sports requirements without effort and without thought, "like (an) automaton".  Gene is indeed "maimed", not in his body, but in his mind and spirit (Chapter 6).