Guy de Maupassant's short story "A Coward" deals first and foremost with the theme of egocentricity vs reality.
The main character, Vicomte Gontran-Joseph de Signoles, is a foundling who came upon great fortune once he is adopted by a powerful family. As an adult, the young Vicomte becomes the epitome of the French dandy: Good looking, well-spoken, charming, seemingly virile, and quite admired. He is so admired that his nickname is "Handsome Signole" among his cronies.
As it is to be expected, all this validation of his beauty and charms has gone straight to the Vicomte's head. Now, with the support of so many admirers, the Vicomte has come up with his own version of himself: In his world, he is the best guy on earth; a version of a musketeer. He has been put on a mission, he believes, to become the ultimate charmer.
However, Handsome Signole's ego becomes so big that he begins to believe in every attribute everyone has ever given him. He is sure that he could take anyone in a duel and win. In fact, he was hoping for the opportunity.
One night, his opportunity materialized: Signole scolded a man who was staring and one of Signole's female friends during a night soiree. The man warns Signole to leave him alone. Signole refuses, and slaps the man in the face. As a result, Signole demands that a duel should occur to satisfy the honor of the lady and his own dignity as a man who has been insulted.
Yet, reality sinks in. It is not the same thing to plan something as it is to do something. When Signole, for the first time, had to retrieve strength from the REAL man he is, he realizes that there is no strength to be found. He is scared. He is a coward. He has lived too long under a fantasy that he is this or that...but he is far from it.
In the end, he shoots himself in a panic, not knowing what would come out of the duel. If he loses, his ego would have been hurt forever (not to mention that he will likely be dead). And the chances for winning? Not very many, from what he can confess to himself.
Therefore. There is the coward: The man whose self-love and egotism blinded him to the fact that he has weaknesses too, that he is a regular man, and that he also can die, be scared, and back out of things. Egocentricity vs. Reality: That is the central theme of the story.