The underlying universal message of a text is the theme.
The theme is a big idea, something that you can learn about life in general.
Here are some examples of themes found in literature:
Love, such as Shakespeare's Romeo and Juliet, a tragic tale of forbidden love with terrible consequences.
Death, The Fault in Our Stars features teenage characters coming to terms with their mortality in the face of terminal illness.
Good vs. evil, The Chronicles of Narnia, C. S. Lewis', follows four siblings who pass into an entirely new world, in which they encounter characters both good (Aslan) and evil (The White Witch)
Answer:
His justifications for killing the old man and his actions throughout the story suggest that the narrator has, in fact, descended into madness.
