I used to have a pretty simple reading of the Cain and Abel story in Genesis 4.
Sin had entered the world and its devastating effects were growing. Cain was jealous of his brother and so murdered him. He was then worried about others killing him and build a city. [Aside: the fact that he build a city suggests there may have been more people around than just Adam's family].
This just illustrates how sin had corrupted people to the point of murder.
However, I now realise that the Biblical account is a lot richer. This comes from reading The Economics of Honor by Roelf Haan, who is heavily influenced by The Meaning of the City by Jacques Ellul. It is also helpful reading Genesis 4 in conjunction with Genesis 11, where through the tower of Babel: humans yearn for autonomy and greatness. This is to be achieved via great productivity and grand constructions. Haan points out how at the time some grand projects were usually built by foreign slaves obtained from conquest and exploitation.
Haan focuses on Cain's
* unwillingness to take responsibility
["Am I my brothers keeper?"]
* his alienation from God and fellow humans and fear ["from your face [God] I shall be hidden. I shall be a fugitive and a wanderer on the earth, and whoever finds me will kill me.”] The fear is similar to modern preoccupations with "security", from homes to nations, being used to justify all sorts of violence.
* motivation for building the city
This is not just a morally and spiritually neutral act. It is an act of rebellion against God.
The painting is Cain slaying Abel by Tintoretto.