Gene decides, after the beach trip with Finny, that Finny is trying to sabotage his academic success. What triggers this is a conversation the two have after Gene fails his trigonometry test because of the beach time. Finny accuses him of working too hard and tells him he needs to lighten up. He informs Gene that he is too competitive, saying:
You want to be head of the class, valedictorian, so you can make a speech on Graduation Day—in Latin or something boring like that probably—and be the boy wonder of the school. I know you.
Gene outwardly denies this, but inwardly admits it is true. He does want to be head of class because it will make him "even" with Finny, who is sure to win all the athletic awards. All of a sudden Gene, remembering that the prep school is a "jungle" where it is impossible to trust anyone, decides that Finny is just pretending to be his friend in order to derail him from success, "he minded, despised the possibility that I might be the head of the school."
All of this tells us much more about Gene's distrustful, competitive nature than it does about Finny. As Gene later realizes, he projected his own fears and "shadow" on his friend. Finny did genuinely want Gene to enjoy life and didn't have any interest in sabotaging him. Gene, of course, feels bad about this later.