The underlying conflict of A Separate Peace is related to Gene's maturity and his choice in which path to take. Two modes of being are available to him through the novel, represented by Finny and Brinker. In the end, Gene realizes that the choice between being like Finny or being like Brinker is a false choice.
Finny can be said to represent youth, innocence and a denial of responsibility, as evidenced in his antics, his attitudes, and his habits. Brinker can be said to represent a rush forward into responsibility and adulthood, as evidenced by his desire to enlist early and in his various leadership positions at Devon.
For much of the novel, Gene follows each friend, going back and forth between enacting Finny's values and enacting Brinker's values. This conflict is one of choice for Gene and also maturity.
Discovering that the choice between Finny and Brinker is a false choice is one major instance of maturity, for Gene and for the novel, as Gene grows up enough to see that he does not need to choose between the path's represented by his friends and role models but must instead choose his own path.
Another significant instance of maturity in the novel comes in the pretext of the story Gene recounts in A Separate Peace.The context of the novel is directly related to maturity. Gene, after years away from Devon, is finally prepared to deal with his guilt over what happened to Finny. Gene has grown strong enough or has matured enought to do this, as an adult. The novel is a story of "coming to terms with the past"; an act of mature reflection and acceptance.
Gene learns valuable lessons about guilt and about himself as he recounts Finny's story at Devon.
"Gene Forrester comes to learn that his war, the essential war, is fought on the battlefield within. Peace comes only when he faces up to the fact."
Finny is mature when it comes to forgiveness, but Gene is more mature in terms of "living in this selfish world" where peoplewhat the best for themselves. He understands very early in life that friendship and kindness doesn't get you everything in life. So, it doesn't matter to him even if he does unthinkably bad things to his best friend.
Also, when it comes to Leper, he was one of the first to enlist for the war. He, in his childish mind, believed that he could handle such levels of violence at such a young age. Unfortunately, he was not mature enough, made the wrong decision, and lost his mind at the end.
hope this helps :)