This occurs when Gene is securing his role as the Assistant Crew Manager. This is not an honorable, but a mocking position for a senior. Gene starts a fight with him. The two end up wrestling through the fight falling into the river.
I think Gene's purpose for responding this way is that he has an over-sensitivity to the word maimed because of it's significance about what he knows he's done to Finny. To have crippled someone else and that someone not knowing is maiming Gene from the inside out.
Gene's spirit has been crippled at the loss of Finny's ability to be himself. Gene is crippled by guilt. Gene is crippled by untruth.