An important theme of the novel The Natural by Bernard Malamud is the difference between material and career success and success as a human being. Although the protagonist of the novel, Roy Hobbs, is a talented baseball player, he lacks the maturity and skill to use his talent wisely. His eventual failure as a player and person is due to character failings, especially the inability to resist immediate gratification in order to achieve long term goals. He is also a morally shallow person, who treats other people selfishly. He sees women merely as means to gratify his sexual desires and men as either obstacles to be overcome or as potentially useful allies.
Your focus, which will determine what you use as a thesis statement, depends on what interests you most about the novel. One possibility would be to start with the Kantian notion that ethical behavior demands that we treat all intelligent beings as ends in themselves rather than as means to ends, and argue that it forms the ethical background for the way in which readers are invited to view the behavior of Hobbs.
A second possibility would be to argue that the way in which Hobbs' attitude towards women, in which he treats them simply as objects to be pursued for physical gratification, leads to his downfall in both parts of the novel, provides the reader with a moral perspective in which self-centeredness and instrumentalist views of others contain the seeds of failure.