Athos spend much of The Man in the Iron Maskcaring for his son, Raoul. His role in the novel is that of a grieving father. He is so involved with his son that he tries to communicate with him through his dreams. Athos feels he has nothing to live for in his son's absence.
We think it is also telling that Athos is the first of the Musketeers to have an explicit falling-out with King Louis XIV. Although we don't see it happen in this novel, we learn that Athos forcefully berated the King for his behavior regarding La Valliere. Clearly, Athos is another type of independent-minded noble fellow who interferes with the King's designs for absolute power. His death, like D'Artagnan's, signals the end of an era.