I agree with accessteacher in that the story has a lot of pessimistic subtopics in the first half to include: Negative loyalty, bad parenting, psychological abuse, battery, racism, prejudice, hatred of others, paranoia, and the topic of arson it itself.
However, when Sarty makes his very difficult self-analysis and recognizes how he had been following a man who is insane, he gives the readers hope:There is a possibility to untangle oneself from the trappings of family or friends who do us more harm than good. It is OK to say "I did not choose this and I am walking away from this". In the end is one's own journey we are responsible for. Everyone else has their own to worry about.
I would want to argue that the main story has a pessimistic tone, whilst the ending is optimistic. This is because the story narrates the way that Abner Snopes condemns both himself and his family to a self-perpetuating cycle of anger and violence. They have to face the results of Abner's arson and this leads them to wander around itinerantly with no hope of change. However, the ending, when Sarty looks at the stars and walks away without looking back gives us hope that he at least can change and escape the influence of his father.