The apocalyptic world that the boy and his father inhabit exists only in our wild imaginations. Speculation about what it might be like is fuelled by stories and movies of terror and man's considered "return" to basic savagery; believing that it is evil that will triumph in the end.
Cormac McCarthy attempts to reveal that "the good guys" might hide away and, themselves , do unthinkable things in the quest for survival but they will never cross the line - cannibalism is NEVER an option, "no matter what."
The snakes' dream has relevance as it is a "real" event that the man witnessed, when he was not much older than his son, and which clearly had an impact on him. Evil deeds are often as a result of a person's reaction to, for example, the evil around them and a person always has a chance of redemption, a change of heart, goodness. Snakes, however, will never be good. Since the beginning of time, snakes have represented slyness, deceit and even brought about the downfall of man when he was banished from The Garden of Eden!
The only way to remove this kind of evil - "the image of it as they conceived it to be" is through - in this instance - burning the snakes. Burning something has a very symbolic meaning because, whilst it signifies destruction, it can also encourage new life. A field after it has burned, grows sweeter grass and so an analogy of burning snakes is perhaps an attempt by the father to remember, in the gloom surrounding them, that he must always have hope of new beginnings. A person must learn from the past which the man burning the snakes did not.
There is no going back. There cannot be a return to the what was. The father( and his son) must make their own future - or at least, a future for his son.