Jekyll and Hyde (The Strange Case of Dr. Jekyll and Mr. Hyde  by Robert Louis Stevenson)
This classic was written 130 years ago. It is available free on Amazon and you can add an audio version for about a dollar. Even in the book, the age old struggle of people to understand and to deal with the dual nature of mankind is acknowledged.
Several interesting features of the allegory should not be overlooked. First, Dr. Jekyll discovers a potion to separate his dark side, in the person of Mr. Hyde, from his good side. But never, is the good independent of the evil. You can have duality, or proof that it exists through Hyde. But there is no Mr. Wonderful in the story.
Second, the narrator is Mr. Gabriel John Utterson, who only briefly is acknowledged to have a dark side. But he is given every detail to tell the story and in the end, gives us Jekyll’s full explanation and rationale, which is very good, in my opinion. Of the three (or four if you separate Jekyll and Hyde) main characters, only he survives – dark side intact.
Third, when Dr. Hastie Lanyon is faced with the reality of the dark side, he is so overcome with the news (provided as proof when he witnesses Hyde’s transformation to Jekyll) that evil lurks in the embodiment of all men that he dies. He even says he will die, and why. This is in spite of the fact that Jekyll explains it all to him. Why did Lanyon die? Because he too had a dark side, but he never believed that he did. He never accepted his true and complete nature. Essentially, his own sin of pride killed him.
For those of us who believe that the basic nature of man is good, this may be a troubling allegory, as it was for Lanyon. But it shouldn’t be. We should not have to separate, indeed we cannot, one nature from the other in an effort to prove its existence. I think that the labeling of the dark side as evil is okay, if that is what you decide to do. But I wouldn’t do that. I prefer to accept my nature as it is. I experience little conflict and move forward with my life – it is what it is.
I do strongly favor the concept of living in the moment, my own version of Carpe Diem works for me. I did notice that in his final confession, when Jekyll is referring to the nature of Hyde (which is supposed to be Jekyll’s own dark nature), he says “…his circumscription to the moment…” in such a way as to condemn it. Embrace it, Harry. It really is all that we have: right here, right now.