Bitterness. I've read this chapter so many times, so I think it surprised me to see this theme emerge, but I don't know why. Bitterness is the result of disappointed pride; I know this first hand. The Lord has had to deal with me and forgive me in regards to the root of bitterness this year, so perhaps because I've finally been able to see my own root, I'm more easily able to see it elsewhere.
Genesis 4 covers the birth of the first two children recorded, Cain and Abel. Then the first child grows to become a murderer. He is punished by God, though shown mercy; and the entire course of Cain's life is altered. Another child is given to Adam and even to replace the legacy that would have been Cain's as the first child of the line of Adam.
The story begins with two young men. Cain is a farmer--a tiller of the ground. Abel is a herdsman--a tender of the sheep. Their parents must have told them about God and their beginnings, because there comes a time that they both come to offer a sacrifice related to their professions.
Now growing up, I had heard that Cain's offering was rejected because it wasn't a lamb, but I'm not sure that was the case. The shadow of the Lamb of God had not yet been set forth through the Israelites system of sacrifices. There is not indication that there had been any such commands yet. However, Hebrews 11:4 offers us a clue. "By faith, Abel offered a more excellent sacrifice . . ."
The fault of the offering did not have to do with the offering itself, but the spirit in which it was offered. Cain offered in pride; Abel offered in faith. Pride was rejected; faith was accepted. When Cain's pride became disappointed, he became bitter. When God challenged his bitterness, rather than repent, he murdered his brother.
Bitterness destroyed the very first family. Bitterness separated Cain from what was left of his family and from God (though his choice, 4:16, "And Cain went out from the presence of the Lord").
The consequences of bitterness left Cain with a life that was void of the things that were initially precious to him. God showed him mercy, in that he lived, but in regards to the profession that had filled him with pride, that life was cursed. He founded a city and fathered traders, nomads, herders, musicians, and metal smiths; but neither he or his mentioned descendants returned to farming. He lived, full of bitterness, but empty of all else; and his bitterness would effect the generations to come.