Brother kills brother. There is no legitimate reason for this act in the text. What is the story telling us? What is it not trying to claim and explain.Obviously, the world is suddenly populated. Cain has a wife. Cain fears that as he wanders those whom he meets will want to kill him. There is no explanation of where these people come from.
Many Christians provide explanations. Adam and Eve, after all, had many other unnamed children (Genesis 5:4-5) however after the death of Abel it seems to imply that the son Seth is a replacement of sorts. (Gen 4:25 "God has appointed for me another child instead of Abel because Cain killed Abel.") There does not seem to be an abundance of children (enough to populate numerous villages) implied by this remark.
I do not think Christians should read their own bible with a different set of demands than they read the sacred writings of other religions. This does NOT mean that I equate the truth of the Bible with the writings of other faiths. I do not. That is why my questions always center on what does the text of the Bible really say. The narrative conveys the rapid decline of the human experiment. Adam and Eve have two sons and one kills the other. There are social statements being made (farmers and herders are represented). There is a psychological/spiritual component (sin is lurking at the door seeking to master you but you can master it) which emphasizes human choice. There is an idea of human responsibility in worship and service of God (If you do well will you not be accepted?). And, of course, there is the age old tragedy of sibling rivalry. Lastly, human anger and jealousy produces murder.
All of us are prone to self centeredness. In our quest for what we want, we do damage to those who somehow diminish our sense of self. The story of Cain and Abel is a reminder that human realtionships and our relationship to God are interconnected. The tragedy of life is how often we choose to harm, even kill, one another. Yet, the Bible wants us to understand that we are connected to those early sinners. Our heritage is tainted by murder and violence. It is a sad fact of life on the planet. The reality of sin is the Problem which we all face. the strories of the Bible never shy away from human sin. These stories are a mirror in which we see our own reflections.