Consider their vocations. One is "a tiller of the ground" and one is "a keeper of sheep." One offers as a sacrifice to God "of the firstlings of his flock and of the fat thereof." The other brings produce.
While this might seem on its face unfair that God should prefer the offering of the shepherd over the offering of the "tiller of the ground," it is really not. It helps if you understand that certain types of offerings were expected. Other types were inferior. Even if one tills the ground, one can trade or purchase, if you will, from the other. The shepherd's best lamb would be pricey, but that is what was expected and the bad brother would have understood this. Thus, it is really not unfair.
Even if it were unfair, the bad brother would be expected to deal with it. Killing one's own brother is not an option, especially when it is based upon vanity (envy).
The bad brother compounds the sin by answering God's question first with a lie and then with a question. "I know not: am I my brother's keeper?'" The fact that the bad brother knows that God knows everything and answers in this manner shows his hubris, or excessive pride. Hubris is generally associated with a character that tries to step beyond his rightful place. In this case, the bad brother is placing himself above God. His response is "smart-alecky."
Part of the murderer's punishment is that "when thou tillest the ground, it shall not henceforth yield unto thee her strength." In addition, he will be exiled not only from God, his homeland, but from any place on the earth. He will remain "a fugitive and a vagabond."
The murderer whines that such a curse will mean that his life is in jeopardy. That is when God places a mark on the murderer, that "whosoever slayeth" him, "vengeance shall be taken on him sevenfold."
Your anthology analyzes the story this and the story this way: "...God drives him out to be a wanderer on the face of the earth. He does not kill him" (48). While this "marks him as a murderer, [it] also protects his life--no one is to touch him" (48).
