Was his action premeditated? What were his thoughts about himself and Finny just prior to the jump? Gene jumps from the limb for the first time without fear, why?
Gene was jealous of Finny, pure and simple. He didn't mean to hurt him the way he did, but he didn't trust Finny's friendship, and he wrongly attributed his own negative feelings and behaviors on his friend.
I could very well be wrong, but I believe that it was because Gene saw Finny as the good in the world and he was jealous of this. Later in the story, Gene dresses like Finny and tells himself that his purpose is to, "...become a part of Phineas." (77 in my copy) I'd love to hear some other input...
