Let's take care of the elephant in the room: Walt's death is a huge bummer. In the chapter leading up to it, Garp is anxiety-ridden about his youngest son, but we learn to laugh it off. "Garp's just being an over-anxious parent," we say. "This is just his imagination running wild on him." Then, in one moment, all of his worst fears are realized.
So how are we—and Garp, for that matter—supposed to rationalize it when our main man's fears come true? Is he right to be paranoid all along? Is his only crime not being paranoid enough?
Garp gives us his answer in the form of The World According to Bensenhaver. Like his note to Helen about becoming "whole again" (14.28), the novel represents Garp letting go of his guilt. Tragedies happen all of the time—there's nothing anyone can do to change that—and because of this, living in fear is never the right choice.
Walt, we hardly knew ye, but thanks for the life lesson.