Gene does not exactly hit Leper; he "shove(s) (his) foot against the rung of his chair and kick(s)," sending Leper onto the floor. He does this when Leper, who is bitter and unflinchingly honest, even about the most sensitive issues, since his return from the military, flat out accuses Gene of being "a savage underneath", and "knock(ing) Finny out of the tree." Gene reacts violently because this is something about which he has been agonizing, the question of whether Finny's fall was an accident, or whether he, in a moment of pique, had purposefully caused it. To hear the words come so directly from Leper - the hidden accusation which is so likely true - is too much for him to handle.
Leper's mother comes in when she hears the commotion, and Gene voices regret. "Too ashamed to leave" when she accepts his apology, he goes for a walk with Leper at her request, and the two continue to engage in strained and awkward conversation. When, trying to find something to talk about, Gene brings up Brinker, Leper explodes in anger, and begins to rant about the hallucinations which haunt him, visions of dismembered and juxtaposed body parts morphing before his eyes. When Leper begins to rave about "a man's leg which had been cut off," the reference in Gene's mind is again too close to the thoughts that torment him, reminding him of Finny's shattered and ruined leg. This time it is Gene who explodes in anger, and he runs away back towards town, leaving Leper alone in the field across which they had been walking, "telling his story into the wind" (Chapter 10).