In A Separate Peace, why did author John Knowles feel it was necessary for Finny to die?

Throughout John Knowles's novel, A Separate Peace, there
are parallels drawn between the internal wars within the main characters and the
conflicts that develop among countries.  In early chapters, Finny  concerns about the
European conflict by stating that the war is really a conspiracy among the world's
leaders.  So, in the same way that Germany and the Allies convinced its youth that they
were fighting for a greater cause, Finny creates

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reverses and deceptions and acts of sheer mass
hypnotism which were so extraordinary that they surprised even
him.

As with the climate of
the World War, also, as Gene narrates,

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Everyone has a moment in history...when his
emotions achieve their most powerful sway over
him.

Thus, at Devon School,
Gene and Finny and others wage their private wars. Seduced by a propaganda film, Leper
enlists; driven by Leper's nonsensical comments, Brinker Hadley, the head student, is
driven to enlist in order to make sense of the war for himself, and Finny creates the
Winter Carnival, an attempt in the winter of despair after the war has started and his
leg is broken, to create sense out of chaos.  Having been coached into participating as
the star of this event, Gene surpasses himself as he fights "that first skirmish of a
long campaign for Finny."  He remarks afterwards that he feels in this afternoon
a "momentary, illusory, special and separate peace."

In a
final parallel between the war and the private war in men's hearts, Gene realizes that
the reason that he jounced the limb on which Finny stood and the reason that wars are
waged is, as Finny remarked about the war "a conspiracy," a conspiracy in the hearts of
men.  For, Gene decides as Leper has called him, that he himself is "a savage
underneath."  Extending this truth, Gene realizes that most men are savages underneath
since wars, private or otherwise, are made "by something ignorant in the human heart." 
As a symbol of the pureness of a some human hearts--for Finny has no envy or jealousies
in him at any time--dies, just as the innocent men die in World War II, victims, like
Finny, of the "something ignorant in the human heart."