Examples of antagonists include Macduff in Shakespeare's play "Macbeth" and Lord Voldemort in the "Harry Potter" series. "Dr. Jekyll and Mr. Hyde" is one example where the book's protagonist also serves as the antagonist.
In "Macbeth," Macduff serves as the antagonist by working against Macbeth after learning Macbeth killed King Duncan. Macduff kills Macbeth at the end of the book.
Lord Voldemort is the main antagonist throughout the "Harry Potter" series, although he appears more frequently in later books of the series. He killed Potter's parents and wants to take over the wizarding world, and only Potter can stop him.
In "Dr. Jekyll and Mr. Hyde," the good Dr. Jekyll performs an experiment to separate his evil side. This results in a split personality, Mr. Hyde, who is the evil version of Dr. Jekyll.