What is the theme in Billy Budd by Herman Melville?

Duty and conscience is a theme in the book. Captain Vere's dilemma—whether to convict Billy and hang him in spite of his sense that the young sailor is innocent—arises from Vere's very nature. Captain Vere is characterized throughout Billy Budd as a man who heeds his duty. Even before Captain Vere appears, a description of the captain by minor character Captain Graveling of the Rights-of-Man anticipates the more central captain's problem: "His duty he always faithfully did; but duty is sometimes a dry obligation " The "dry-ness" of duty is in its disconnection from feeling or intuition: duty is intellectual rather than emotional. And Captain Vere is described as possessing "a marked leaning toward everything intellectual," and "never tolerating an infraction of discipline." He adheres to the law and expects his men to do so as well.