How does the revelation at the end of the story contribute to the story’s theme?

This story is really about hatred and racism. The irony being that it is Armand himself with the mixed ancestry. The incredible fear that Armand harbors about how Desiree is supposedly mixed leads him to think that marrying her, being with her, giving her the attention that he did, and having a child with her has completely tarnished the name of his family and his home. He refuses to believe Desiree, despite her desperate attempts that almost conclusively show that she is indeed of pure white heritage (at least insofar as her skin is even whiter than Armand's). He falls into a state of complete indifference - his hatred is so pure, so raw, and so great that nothing, until he reads his mother's letter, could convince him that he has black blood, and that he was the reason why his son has a darker skin complexion. Armand is blinded by his emotions - first of love for Desiree and then of hatred of the black race - and both emotions are so intense that it is not until the end that he realizes how wrong he is about Desiree and about his own heritage.