In "Barn Burning," why does Sarty finally defy his father and try to warn the De Spains?
As Sarty and his family are forced to move on yet again thanks to his father's aggression and his acts of arson, he at first rather naively perhaps believes that this time his father might change and that he will abandon his barn burning activities, giving his family the chance to stay in one place and be accepted by the local community. Note how Sarty hopes that his father will feel the "peace and joy" that he feels now that they have been moved on:
Maybe he will feel it too. Maybe it will even change him now from what maybe he couldn't help but be.
Note how we are presented with Sarty's belief that his father can change and he can exit the spiral of destructive tendencies that he is clearly on, and which he has dragged his family on to as well. Even when Major de Spain says he will charge Abner twenty bushels for the damage to his rug, Sarty still hopes against hope that this will be resolved peacefully:
Maybe it will all add up and balane and vanish--corn, rug, fire; the terror and grief, the being pulled two ways like between two teams of horses--gone, done with for ever and ever.
Note the internal conflict that Sarty reveals he is facing as he struggles between loyalty to his father and his deeper sense of what is right. However, when it becomes clear that his father will not let the supposed "insult" of Major de Spain rest and will burn yet another barn, Sarty realises that this cycle of violence and aggression will never end, and he must resolve the conflict within himself by doing what he knows to be right, even if that means betraying his father. Thus it is that Sarty decides to tell Major de Spain of what his father is planning to do, simultaneously freeing himself from the oppression of his father and following his own conscience.
At first, Sarty hopes that the fires will end. Unfortunately, Sarty finds out that this was not to be so. Abner begins to set ablaze his next barn. This time his father breaks his own moral code by not sending anyone to warn. Sarty pleads "Ain't you even goung to send a [slave]? At least you send [a slave] before!" Sarty's young mind is made up right then and there. He intends not only to extinguish the fire but also to extinguish the family connection. His act of warning the de Spain despite knowing fully well that he will incur his father's ire sends off the message loud and clear- that he is not going to be an accessory to his father's crime anymore. Fortunately, unlike his elders, Sarty is not corrupted enough to let go of his morality. The "pull of blood" is not strong enough reason to corrupt Sarty's principles. He stands by his principles and breaks free from his family's influence as epitomized by Sarty's breaking loose from the strong grasp of his mother's hand. Just as Sarty is able to let go of his mother, he too is able to let og of his ties as he strives to pursue nobler goals. His father's offense and his youthful sensitivity lead Sarty to his noble decision to warn the de Spain's. Sarty chooses not to look back to his family's painful past and move on without them.