Throughout the whole novel it seems that Gene has a very low self-esteem. He is not very happy with his life because he is always in Finny's shadow. Everything he thinks he's good at Finny has a way of showing him up and doing better than he does. In the beginning of the novel Gene is a very quiet person and he wants to succeed in life, an example of this is the way he wants to become valedictorian. Moreover, Gene's anger toward Finny builds as the novel goes on. An example of this is how sometimes Finny pulls gene away from his studies for whatever reason and then Gene fails the exam that he had to study for.
Another example is how Finny would tell the truth about Gene's height if he fibbed to someone, making him an inch or two taller. This really upset Gene because all Gene wanted was to feel a little bit better about himself by making him look taller and then Finny crushes it by exposing the truth making Gene's self-esteem go down even lower. Gene views Finny as a superior, and in a sense wants to be him. An example of this is when Gene put on Finny's clothes while Finny was in the hospital due to his fall from the tree; Gene wants to be Finny, he wants everything that comes with being "Finny," the athletic gifts, being popular, and being comfortable with himself, because he knows that if he can do this then he will live a much better life. Gene's anger finally got to the point where he couldn't take it anymore, so he "pushed" Finny off of the tree. His jealousy motivated him to commit this act against his best friend.
Gene spends the whole novel believing that Finny is his enemy, however, by the end of the novel, the "grown up" gene realizes that Finny was never the enemy, but the real enemy was himself.