The question of realism in the ending of William Faulkner’s Barn Burning is one both of the character of Sarty and Faulkner’s portrayal of the Old South. The fictional landscape Faulkner creates, often referred to as “Southern Gothic” involves extremes of inbreeding, miscegenation, drunkenness, perversion, and complex but tightly knit extended families. It’s unlikely that the inhabitants of every southern town were, on the average, this bizarre, and engaged in the activities Faulkner describes, but on the other hand, one could argue that there were individual examples of such situations, and just as newspapers choose to write about what is uncommon and therefore newsworthy (“dog bites man is not news; man bites dog is news”), so too can Faulkner legitimately choose to portray extreme cases.
Abner Snopes’ resentment of wealthier and more successful people leads him to vandalize the de Spain’s property. Since even today, jails are filled with such people, this can hardly be considered improbable. Sarty’s choice takes considerable moral courage, but moral courage on part of the young is not impossible. While the story isn’t unrealistic in the sense of completely impossible, or the characters being unbelievable, eventually the point is more about how the history of slavery and class oppression effected the moral nature of the south than a realistic novel about specific characters.