Here is the passage in question. As we see, Cain kills his brother, God confronts him with it, then the mark is applied:
And Cain talked with Abel his brother: and it came to pass, when they were in the field, that Cain rose up against Abel his brother, and slew him.
And the Lord said unto Cain, Where is Abel thy brother?
And he said, I know not: Am I my brother’s keeper?
And he said, What hast thou done? the voice of thy brother’s blood crieth unto me from the ground. And now art thou cursed from the earth, which hath opened her mouth to receive thy brother’s blood from thy hand; When thou tillest the ground, it shall not henceforth yield unto thee her strength; a fugitive and a vagabond shalt thou be in the earth.
And Cain said unto the Lord, My punishment is greater than I can bear. 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.
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. (Genesis 4:8-15)
As we see, Cain was cursed because of his sin, not because of the mark. Specifically, his curse meant that farming would be more difficult, and he would be an outlaw wherever he went. Cain brought this upon himself.
When Cain said he did not think he could bear the consequences of his sin, God placed a mark on him to help him out. We don’t know what the mark looked like or where it was put. (It was certainly not a darker skin coloring, as some used to teach.) The mark warned people of severe repercussions should they seek to harm Cain, and so Cain was protected.
We don’t usually think of God as one who would help out someone who murdered his brother, do we? Yet capital punishment had not yet been instituted at that time. Perhaps because Cain got away with murder, murder would increase throughout the world until there was only one righteous man standing – Noah. Then God sentenced a world of murderers to death with the Flood. Only after Noah had set foot on dry ground again did God institute the death penalty for those who shed his fellowman’s blood (Gen. 9:6).
We, too, are cursed because of sin. Our sinful choices tear families apart, destroy friendships, affect us physically with headaches, nausea, and sleeplessness, bring on psychological effects with guilt, depression, and anger, and the list goes on.
Thankfully, Christ became a curse for us (Gal. 3:13). On the cross, all the blame for our sin was laid upon him, and he paid the ultimate price. Through his death, burial, and resurrection, we are set free from many of the harmful effects of sin. Families are reunited through humility and forgiveness, we sleep again because of peace with God, and our guilt is gone.