Cain commits the murder of his brother, Abel. Instead of repenting of sin his heart hardens and leads to more sin. This is the danger of not obeying the Gospel (John 12:42-43). We are changed in one of two ways when we hear the truth. Either the heart is molded by the truth or it hardens. There is no middle ground. Cain hardened and his crimes extended from improper worship to murder and lying to God.
God then gives His sentence. God uses object lessons often to teach truth, that is, He uses visible effects to teach others. The smoldering ruins of Sodom and Gommorah come to mind (Genesis 18:19 Abraham would take that lesson and use it to teach his house about the sins of Sodom and Gommorah while the smoke still rose from the cities of the plains.). This may be the reason that God didn’t use the death penalty in Cain’s case. Cain, the vagabond, would be a reminder for centuries of God’s punishment on the evil doer. Having Cain around with God’s mark on him was a deterent to others who would settle their disputes in this manner.
Why would sevenfold vengeance be taken on the one who killed Cain? Because God had exercised His sovereign authority and had ruled on the punishment of Cain. No one else had the right to do so once God had settled the matter. It would be presumptious to seek to override God’s decision. The mark God placed on Cain must’ve been much like a king’s seal, indicating the authority of the king.
“Cain went out from the presence of the Lord.” From the beginning sin has always separated God and man (Isaiah 59:1-2). Ultimately the separation caused by the sin of man is an eternity spent in hell (Rev. 20:15). There are many who believe that God is just too loving to condemn a soul to eternal hell. However we have already seen twice in the sacred text that God enforces His laws. He has already removed man from the tree of life. If God says that an eternal hell awaits the sinner we had better believe it. Notice 2 Thessalonians 1:7-9; Matthew 10:28; Matthew 25:46.