If Gene is afraid to jump out of the tree, why does he do it? What does Gene realize after he jumps from the tree?

To start off their meeting of the Super Summer Suicide Society, they decide to do jump off the tree together. Once they are on the tree, Gene "jostles" the limb, and Finny loses his balance and falls; Gene, unshaken, jumps into the river, seemingly without remorse or concern for his friend.