The historian who writes about decline and fall will dwell on the misdeeds of statesmen; the novelist focuses on family. In “The Complex,” the architect of the Chopras’ ruin is S.P.’s son Laxman, a coarse, grasping status seeker who, at the end of his life, becomes a rabble-rousing Hindu nationalist in the BJP, the political party of the current prime minister, Narendra Modi. The story relates Laxman’s making as a demagogue, dwelling on the years in which he used his extended family as a kind of training ground for his megalomania. To Mr. Mahajan, a country’s corruption begins at home.