Right before Gene jiggles the tree, he shows what seems to be a changed mindset, but in fact, he displays a fixed mindset. For some time, he has believed that Phineas was in competition with him and was trying to distract him from studying so that he won't become class valedictorian. He believes that all Finny's friendliness and activity are a ruse to make him stumble academically. He notes,

I had detected that Finny's was a den of lonely, selfish ambition. He was no better than I was, no matter who won all the contests.

It is, in fact, Gene who is the den of selfish ambition. He feels a rivalry with Phineas that leads him to fear that he will be beaten and come in second. He knows that Finny is the most athletic boy in the class and feels that the only way he can stay even is to be the best academically. He projects his own ambition and desire to win at all costs on his friend.

Gene seems to undergo a change of heart right before they go out and decide to jump together from the tree. Gene realizes that Phineas was never trying to undermine him or win, because Finny doesn't operate that way. He doesn't care about winning the way Gene does. Gene realizes that

there never was and never could have been any rivalry between us. I was not of the same quality as he.

Here, it seems as if Gene is undergoing growth, because his mindset has changed. But then Gene thinks, "I couldn't stand this." The "this" that Gene can't stand is ambiguous, but when he shakes the tree so that Finny falls, it seems that what Gene couldn't stand was the very fact that Finny was a better person deep down inside—of a better "quality" than him. Gene shakes the tree because his sense of rivalry and jealousy are still acute. If he can't have the inner goodness of Finny, Gene seems to want to equal the score by making sure Finny's body is shattered.

Is it not until later that Gene will grow as a person. He sticks to the adolescent mindset of competition and rivalry at this time because up until he jiggles the tree, nothing has yet happened in his life to propel him to grow up and stop fixating on himself.