God warned Cain to rule over his sin (v. 7). Sin, however, took over Cain and he killed his brother. The Bible does not offer us his exact motivation but we need none. Cain’s own sin acted out and he remains guilty for his murder.
- Let us learn well how sin can so quickly degenerate (vv. 8, 9).
In contrast to his parent’s own responses to their sin, Cain exhibits how quickly sin can degenerate. His parents eventually admitted to their sin (after making excuses). Their son, however, lies about the whereabouts of his brother (v. 9): “I do not know; am I my brother’s keeper?” He has actually become his brother’s murderer.
We read nowhere of Cain’s repentance; we find nothing of Cain’s sorrow. We witness a defiant self-pitying sinner more concerned about the consequences of his actions than about the wickedness of his own transgression.
- Let us be quick to never charge God with injustice (vv. 10-14).
When confronted with his sin (v. 10) and cursed for what he did (vv. 11 -12) he complains about the punishment of his sin rather than addressing the depravity of his offense. His parents never complained about what happened to them; Cain thinks God went too far. Rather than confessing that God has justly dealt with him (surely, not as his sin deserved), Cain charged God with being too severe: “My punishment is greater than I can bear.” (v. 13)
Are we like Cain on this matter? Do we complain that the Lord has been too severe with us? Do we not imply that He is not wise, good, and just? May we always confess that He deals with us not as our sins deserve.
- Let us notice that God’s restraining grace preserves human society (v. 15).
There is an irony in Cain’s fear of being killed. He who killed his own brother fears being killed by his own kin. This fear assumes Adam and Eve had many other children and their children’s children began to rapidly multiply and fill the earth.
God’s first statement, “If anyone kills Cain, vengeance shall be taken on him sevenfold.” means God will ensure that complete justice will be meted out. The statement “shall be taken on him sevenfold” means vengeance will be full like in Prov. 6:31 which says that if a thief “is caught, he will pay sevenfold.” This would ensure that murder would not obliterate mankind.
Regarding the “sign” or “mark”, we cannot be certain what it was except we can clearly understand its purpose. Whatever it was, the mark was to somehow prevent Cain from falling prey to another.
Cain disavowed his innocent brother: “Am I my brother’s keeper?” (v. 9) God would act with compassion against this unrepentant murderer to spare him; in a very loose sense, God would be Cain’s keeper and protector. He is treated far better than his sins deserve!
- Let us observe the saddest effect of rebellion against God (v. 16).
Cain complained that he would be hidden from the Lord’s face (v. 14). As of this moment, God was still communicating with Cain and the appointed sacrifices would have enabled him to draw near. But the saddest and the most grievous effect of sin is to be away from the “presence of the Lord” (v. 16). As one commentator has noted: “The story that began with the attempt by Cain and Abel to draw near to God through sacrifice ends in Cain’s “leaving the LORD’s presence” and living “east of Eden,”…”
Sin always separates us from God and unless God acts to restore this estrangement, we will forever be separated from Him. But thanks be to God for the gospel. God’s good news is that we can draw near to God through Jesus Christ who died for our miserable hell deserving sins. Through the shedding of his blood for our sins, we can enter into God’s presence if we have received and rested upon Christ alone for our forgiveness and salvation.
 Gordon J. Wenham, Genesis 1–15 (WBC 1; Accordance/Thomas Nelson electronic ed. Waco: Word Books, 1987), 110.