The allegory of maturation
The story is a bildungsroman, which means a coming of age story. Therefore, the plot is an allegory about growing up, and about what it feels like to bloom into your full adult identity as an independent agent. The story is expedited by Billy's humility and curiosity, because he can learn more quickly and when he fails, there are helpers standing by to get him back on track.
His final sign of maturity is his bitter weeping at the end of the novel. His full blown episode comes about when he has to face the evil within him. He realizes that he is mortal and fallible, and that the emotional recompense for evil is a dark misery that drives him to absolute despair.
This constitutes adult knowledge. He is accepting responsibility for himself as an agent in the world. He understands evil, and just like his primordial forefathers, he learns that one mistake can cost everything. The stakes of life are high, but he understands that, so he can be called "A well adjusted adult." The story is complete.
The motif of violence and wounds
The fact that such little violence occurs in this novel is astonishing for anyone who has read McCarthy's early novels. This means that the subject of the novel is probably non-violence, and indeed, the main character trait of the boy is that he won't shoot the wolf, but rather goes out of his way to get the wolf back to its home.
That means that when violence does occur, it's highly symbolic. There is a motif of violence that basically depicts two types of violence, random violence and incurred violence. Boyd invokes the violence that takes his life, because he doesn't learn from his mistakes, and he doesn't have enough emotional control or friendliness to avoid personal violence from others. Then there is the senseless, unneeded violence of the Mexican bandits. This is malice, or mal-intent. Billy is a hero because he doesn't put more violence into the world, but protects and restores animals that are wounded.
The allegory of the bad brotherhood
McCarthy invokes another sacred archetype, the Biblical archetype of Cain and Abel. There are many reasons why brothers might not get along, but the idea is that their differences represent different approaches to life itself, forming the young men into beings that aren't compatible. The meaning of the image is the ethical principle that some approaches do better than others. In Cain and Abel, this leads to the ironic death of the better brother, but McCarthy tells the other version of the story where the bad guy dies instead. Not that Boyd is necessarily evil, but his basic attitude toward life gets him shot twice and eventually killed.
The motif of the Mexican wilderness
The Mexican flavor of the novel has been commented on by critics, but basically, Mexico represents the foreign land of the unknown. This archetypal image is a subtle expression of Joseph Campbell's hero monomyth. The young hero integrates himself to society by traveling into an unknown place and conquering trials and learning from them. Mexico represents the potential of growth by the mastering of darkness and the unknown.
The symbol of the father image
Billy's perspective is enlightened because, as Carl Jung might say, Billy has integrated his father into his own identity by emulating the man's careful approach to life. The image through which Billy perceives his father most clearly is the image of his father, silently stooped over, trying to fashion a trap. There's a wolf that has been stealing cattle from the herd, and Billy sees that by thinking clearly and acting with integrity, his father manages to solve the problem. Billy then takes his father's approach one step further and develops an identity built around the idea of non-violence itself, sparing the she-wolf from death.
