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.