13 ¶ And Cain said unto the LORD, My punishment is greater than I can bear. 14 Behold, thou hast driven me out this day from the face of the earth; and from thy face shall I be hid; and I shall be a fugitive and a vagabond in the earth; and it shall come to pass, that every one that findeth me shall slay me. 15 And the LORD said unto him, Therefore whosoever slayeth Cain, vengeance shall be taken on him sevenfold. And the LORD set a mark upon Cain, lest any finding him should kill him.
Like a modern day criminal Cain is more concerned about the severity and supposed unfairness of his punishment than he is about his sin. While Abel is dead and Cain is living he still forgets what he has caused and whines about how his very life is in danger. As Matthew Henry noted, people like this go from presumption to despair. Many who come to the front of a church to prayer the prayer they are told leads to salvation often are more sorry for the punishment they have received for a wasted life than they are for the sins they have committed. Cain condemns the righteous judge rather than being sorrowful for the unrighteous sin.
Revelation 16:11 And blasphemed the God of heaven because of their pains and their sores, and repented not of their deeds.
The Christian is called to a different attitude toward his or her sin.
Psalm 32:5 I acknowledged my sin unto thee, and mine iniquity have I not hid. I said, I will confess my transgressions unto the LORD; and thou forgavest the iniquity of my sin. Selah.
1John 1:9 If we confess our sins, he is faithful and just to forgive us our sins, and to cleanse us from all unrighteousness.
Here, a point needs to be made about dispensations. God has chosen to deal with men in certain ways that are different from age to age. Note here that revenge on Cain for Abel’s murder is forbidden by God. Note here, though, how that changes after the Flood.
Genesis 9:6 Whoso sheddeth man’s blood, by man shall his blood be shed: for in the image of God made he man.
The mark of Cain identifies him as under God’s protection so that his brethren, and they really would be his unnamed brothers, do not take revenge upon him. With Eve’s childbirth multiplied as part of the curse on her (see Genesis 3:16) there could have been dozens, hundreds, or even thousands of children by this time bearing children of their own, for we don’t know how long this was from creation or exactly when Eve began conceiving.