The idea of the "fallen man" comes originally from the Bible. In Genesis, God places Adam, the first man, and Eve in the Garden of Eden - in paradise. When Adam follows Eve into sin by eating the fruit of the one tree that was forbidden by God, Adam gains knowledge of good and bad, knowledge of sin, recognition of right and wrong. Adam falls out of grace with God through this original sin, and is given a future filled with work and hardship to survive.
Gene, at times envious, at times threatened, at times angry with Finny, falls away from the friendship he could have shared with Finny throughout their time at Devon and beyond because of his own actions. The fault for his mental and emotional punishment is his own, as it was for Adam.