What is the theme in Peril at End House, by Agatha Christie by Agatha Christie?

Evil is another theme in the story. The novel draws clear lines between good and evil. Evil is portrayed as something tangible in the novel, something that can be felt and responded to by most people. Ellen says it outright, that she feels that End House has an evil atmosphere. The people of the town labeled Nick's grandfather as evil, branding him Old Nick. The branding of Nick by the same reference to the devil is an indication that the presence of evil was tangible on her, even as a child.