The real turning point in this excellent story for Sarty comes when he sees his father going to commit arson once more and how he responds when Sarty asks him if he is going to send a messenger to let the family know that their barn is burning. Before this, even though he wishes to prevent himself, he finds that "the old blood which he had not been permitted to choose for himself" directs him to be obedient to his father and to do what he is ordered to do, no matter how much he wishes that he doesn't have to.
It is only when Sarty asks his father if he is going to give the family any warning, and he is punished as a result, that something seems to break in him and he is freed from this bad blood and able to do what he wants to do. Being confronted with his fathers face "in breathless and frozen ferocity" is clearly something that enables Sarty to see how his bad blood is something tha will only bring him sadness and grief, and therefore gives him the strength to break free from it. From this point onwards in the story he is opposed to his father and in conflict with him, breaking free from his family to alert the de Spains as to his father's intentions.