Adam and Eve at first bear two sons - Cain and Abel. These two offer sacrifices to God, who smiles upon Abel's offering but is not pleased with Cain's offering. In a jealous rage, Cain kills his brother. When God discovers the murder, he banishes Cain to a life of wandering - he can no longer work the land to get food, so he becomes a nomad. Cain worries that someone will kill him for what he has done, but God says that anyone who kills Cain will be punished seven-fold.
This story is the crime and punishment motif. It begins with Cain offering disrespect to authority (God) by offering "fruit of the ground." I take this to mean he just picked up some fruit off the ground. Potentially it was rotten or had worms in it, but at best he didn't toil or sacrifice in order to give this offering to God. Abel, on the other hand, offered the firstborn of his flock and some nice juicy fat - a real sacrifice. Was Cain lazy? Selfish? Or simply disrespectful? When God rejected Cain's sacrifice, Cain held a grudge. He then committed murder. And murdering his own brother makes the crime even darker. Finally, he lied to God when asked if he knew where Abel was.
Clearly, Cain had the heart of a criminal. His crime was even worse than that of his parents. So why did God say that if anyone murdered Cain the murderer would be punished seven-fold? Was that to show the mercifulness of God? Or was it to explain the existence of nomadic peoples (Cain's descendants)? Also, given that Cain was an evil person, what does that say about the Hebrews' view of the nomadic peoples that descended from Cain? Were they viewed as evil as well?
One thing that struck me is that God does not appear to be omniscient in this story. He does not know where Abel is at first, and thus asks Cain. Or was he simply setting Cain up for a lie? That seems unmerciful, and contrasts with the mercy shown when God says that anyone who murders Cain will be punished seven-fold. Did God have human-like flaws like inconsistencies? Unlikely. Therefore I'll have to assume that God is not omniscient in this story.
Another thing that stuck me is the assumption that there were enough people out there that someone would murder Cain if he wandered away from his family. And whom did he marry? This wife of his was not mentioned in the genealogy of Adam and Eve (which occurs at the end of the chapter). Though women are notably absent from the genealogy. But if he did marry his sister, what does this say about her, that she would marry her brother's murderer? I feel that the story means to imply that there were more than just Adam and Eve's family, but that contradicts Genesis 3:20 which said that Eve "was the mother of all living." I am tempted to consider each story as partly independent from the others.