Why does Leper accuse Gene of causing Finny's fall?

Leper is definitely changed; he is very unpleasant and bitter with Gene, and seems to have been scarred in some way by his time in the service. Leper accuses Gene of causing Finny to fall out of the tree, and reveals that he left the service because he was about to be discharged for mental health reasons. Leeper is dealing with his own pain by causing Gene pain.