The primary theme throughout the entire novel is that of good versus evil. Hercule Poirot has an inner struggle with good versus evil and so do each of the characters that are involved in the story as the murder plot unfolds. One of the primary struggles with good versus evil is with the Styles guesthouse. Hastings, Poirot and several of the guests question whether or not the house has an evil environment because of the murder that took place there decades before. As more guests at the guesthouse start to die, it makes it look like the house is evil.