What is the irony in the short story "Mateo Falcone"?

In this story, it is Fortunato’s lawful act that ends up condemning him to death.  And it is this lawful act that likewise soils him in his father’s eyes.  The boy is described as showing “signs of a promising disposition,” though what this necessarily means to his father Mateo Falcone, a man quick to anger, with a mysterious past but an honest reputation, is unclear. 

One day, when Fortunato’s parents are off on the plains and the boy is left at home alone, a wounded man, a criminal, limps down the road toward the house.  After much agitated arguing, he bribes the boy into hiding him in a haystack from the pursuing authorities.  When the authorities catch up and search the house, they in turn argue with Fortunato, asking him to reveal the whereabouts of the man Gianetto, wanted for thievery, assault, and murder.  At first Fortunato remains true to his word and reveals nothing of Gianetto’s hiding place, but the adjutant Gamba offers the boy a fine silver watch in return for the information.  After much hesitation Fortunato is bought out, and reveals Gianetto to the authorities.

The boy, in simplistic terms, has done the right thing – he has handed a criminal over to the authorities.  He has done the honest thing, from the perspective of the law.  However, from his father’s perspective, he has dishonored his name.  “This child is the first traitor of his race,” Mateo intones, and is furious at his son’s actions.  He has gone back on his word, and traded a man’s life for a shiny silver watch.  Mateo smashes the watch against a stone wall in his anger; Fortunato has now freely handed Gianetto to the authorities, and has nothing to show for it, though the return was at first expensive.  For this, Mateo can never forgive his son, and deals him the justice he sees fit for Fortunato’s offense – a life for a life.

So the irony is condemning:  Fortunato is bribed into doing the right thing according to the law, and yet this honesty, so worthy of praise according to Gamba, ultimately costs the boy both his bribe and his life.  From another perspective, we have Mateo, a man who considers family ties and tradition to be extremely important, who is led to kill his own son rather than compromise on his values of honor and loyalty.  His own standards of justice lead him to abandon that which had given him so much pride:  his only son.