Why did Gene lash out at Quackenbush?

Gene goes to crew practice, which is run by Quackenbush, the uniformly disliked crew captain. Gene is assistant captain, and not on the team; Quackenbush immediately challenges him, not trusting Gene because of his non-participation in school sports other than to manage and help out. They have a fight, and both tumble into the water; Quackenbush tells him to get lost, and he does.

In this chpater, Gene identifies with Finny to the point of taking on Finny's struggles and sympathizing with him by sharing Finny's physical limitations. Gene feels that in his argument with Quackenbush, he is somehow defending Finny, though Finny is in no way involved; Gene feels that he has become "Finny's defender," and seems to take the role very seriously. Perhaps out of guilt for hurting Finny, Gene sympathizes with him by not participating in sports, as if he had a shattered leg as well. Perhaps Gene wants to take on Finny's burdens, out of guilt for wronging his friend; and perhaps it is part of Gene's denial of his wrongdoing, another theme of the book. After the accident, Gene's jealousy and suspicion disappear almost completely, as he begins to bond himself to Finny; and, also out of a sense of guilt-born obligation