In the novel A Separate Peace, by John Knowles, Gene and Finny experience many life altering experiences, not only because of the rivalry between the two boys, but also because World War II is on the horizon.
Gene, being the classical introvert and somewhat of a misfit at Devon, develops a friendship with Finny that grows in its intensity and culminates in an unhealthy jealousy of Finny's talents and abilities. As the novel progresses, the boys create a suicide pact society called the Super Suicide Society of the Summer Session. This pact requires each member to leap from a very high tree into the river.
Regardless of their ever growing friendship while at Devon, Gene becomes more and more obsessed with his need to be like Finny. He is jealous, bitter, and becomes quite resolute that he will surpass Finny in popularity and athletic prowess. Sadly, when the boys are jumping from the tree into the river, Gene is compelled to jiggle the limb when Finny is trying to jump. This causes him to lose his balance, and tragically break his leg. This, of course, fulfills Gene's need to surpass Finny, because the break is so bad as to destroy Finny's participation in sports.
Why did Gene do this terrible thing to his best friend? That is what we need to explore. If you want to divide the book into the realms of good and evil, it is quite simple. Gene, though a nice boy in the beginning of the story, becomes almost possessed by jealousy. He acts upon this raging jealousy, which we can categorize as evil. After all, Finny never meant Gene any harm.
Later, when Gene tries to confess his deed to Finny, Finny refuses to believe that this could even be true. Finny's sense of loyalty to Gene represents goodness as an opposite to Gene's evil.
The subject matter of World War II also comes into play, as the war continues throughout the 1940's. Hitler and the Nazis are regularly portrayed as evil, hence mirroring Gene's feelings of hatred borne of jealousy and envy towards Finny. We can compare Finny to the Allied Forces, which stand for goodness. Think about why the author inserts the war into the novel in the way that he does, Leper's experiences in battle, and Brinker's unrelenting pursuit of Gene. These elements all contain the traits of good and evil, and each boy represents some level of each. It is your proposition to prove how and perhaps to what extent the good and evil exists at Devon. Is Devon like the setting of the burgeoning war? In what ways does the concept of war, which may be construed as a necessary evil reflect the characters in the story?
Regardless, the story does not end well for either boy. The boys, led by Brinker, conduct a mock trial to convict Gene of his misdeeds towards Finny. Finny is overcome by sadness and betrayal, and he makes the choice to leave the room. He is understandably upset, and is in a hurry to get out of the situation, and he trips, falling down the treacherous staircase, breaking the same leg yet again. Tragically, Finny dies during the operation to fix his leg. Does this make Gene's initial act of violence even more evil?
Prior to his passing, Finny and Gene spend a moment together, as Finny lay dying. He embraces his friend, and states that all the anger, envy, and petty jealousies die as Finny dies. Essentially, Finny is left to be an almost angelically good figure as he smiles up at Gene while dying, thereby showing his innate goodness in forgiving Gene. Gene seems to come to terms with his sins, but has to live with his choices for the rest of his life. Think about the ending. Does it declare in any way that good wins out over evil deeds, or is it the opposite, because the goodness dies? You have asked a powerful question, and A Separate Peace lends itself perfectly to the dichotomies of good and evil.