This novel shows adolescent insecurity harming others when the author has Gene cause Finny to fall from the tree and hurt himself so badly.
Gene is somewhat jealous of Finny because Finny is so good at sports, so self-confident, and so charismatic. Meanwhile, Gene doesn't really have an identity. He doesn't have anything that really makes him feel good about himself. He is smart, but that doesn't usually make adolescents feel all that secure.
Because of this, he shakes the tree while Finny is getting ready to jump and he hurts Finny badly.
There are two more characters that demonstrate this idea in the novel: "Leper" Lepellier and Brinker Hadley. Leper is a quiet teenager who keeps mostly to himself. However, when he sees the recruitment video of soldiers on cross-country skies, he immediately enlists. Once in the Army, he loses all sense of self. His psyche cannot handle the sudden onslaught of discipline and rigor, monotony and hostility. He suffers a nervous breakdown and leaves the Army without permission. When Gene goes to visit him, he finds a very different friend. Leper tells him of life in the military, of his hallucinations and daydreams: results of his overwhelmed mind. Gene does not comprehend this change; it frightens him too greatly. He realizes how fragile his own identity is, which causes him to shriek at Leper, telling him he's crazy and simply leaving him crying in his sitting room, hurt by someone he considered a friend.
Brinker too lashes out at friends due to insecurity. The mock trial scene shows this. His own unhappiness at being unable to please his father, coupled with his sudden obsession over Finny's denial about enlisting causes him to latch onto Gene. He cannot let go of the idea that Gene pushed Finny out of the tree. It suddenly becomes his mission to prove Gene's guilt. He stages a mock trial, bringing in witnesses and eliciting testimony. It is this trial which sends Finny running down the steps, where he slips & breaks his leg again. Thus, Brinker's insecurity causes harm to both Gene and Finny.