Here is another classic you should add to your collection, if you have not done so already. Robert Louis Stevenson’s Strange Case of Dr. Jekyll and Mr. Hyde.
The reason of the story is for Stevenson to caution the reader that no matter how good you are, you always have a different side to you. Same if switched. Dr. Jekyll drank a potion that took his other side, the side that he never knew he had, and opened it up to society.
He may have been a successful, well-mannered man, but he had a dark side that came out. That needed to come out. Dr. Jekyll realized that Mr. Hyde is his alter ego. There were times he was able to control him, until unfortunately it became to much.