Cain did not ruler over the sinful desires in his heart. He took his brother out to a field and murdered him. The Lord, in a moment reminiscent of Cain’s parents’ sin, asked him where his brother was. Cain answered with a rhetorical question that did not mask what he had done. God told Cain what he has done and sentenced him to be a wanderer on the earth. Cain’s protest opened the door for God to demonstrate His grace. He placed a mark on Cain’s forehead so that any who saw him would know not to attack him. Like other details in this passage we do not know what the mark was or what it looked like, but it did alert any who saw Cain that they were not to shed his blood.
Cain settled in the land of Nod and became a city builder. He knew his wife and had a children who had children. In spite of his sin, he fulfilled the creation mandate to be fruitful and fill the earth. One of his offspring Lamech followed in his murderous footsteps. He took two wives, killed a man, and said that he would be avenged even greater than Cain would be if someone struck him.
Genesis 4 illuminates the dark reality of life in a fallen world, but there is a glimpse of hope at the end. Eve gave birth to another son and names him Seth. She understood this to be God giving her a son in place of the one that she lost. God promised one who would crush the head of the serpent. As this son was born, Adam and Eve must have wondered “is this him?”