Ge 4:13 And Cain said to the LORD, "My punishment is greater than I can bear!

14 "Surely You have driven me out this day from the face of the ground; I shall be hidden from Your face; I shall be a fugitive and a vagabond on the earth, and it will happen that anyone who finds me will kill me."

15 And the LORD said to him, "Therefore, whoever kills Cain, vengeance shall be taken on him sevenfold." And the LORD set a mark on Cain, lest anyone finding him should kill him.

16 Then Cain went out from the presence of the LORD and dwelt in the land of Nod on the east of Eden.

Cain was more concerned about his punishment than he was with his sin. Unrepentant people are like that. They bewail their punishment but not their rebellion against God.

His conscience was actively working, he knew that justice was out to get him. But God, in grace, placed a protection over Cain. It is useless to speculate about what the sign was. God never told us what it is in His Word.

Cain's judgment and preservation was typical of what would happen to the Jewish nation later on.

Moses is teaching them that even though they may not keep God's covenant, God, in grace, would still preserve them as a nation. Israel became disobedient, murdered the prophets that God sent them and yet God still preserved them. Israel would continue to exists.

Ultimately, this was a picture of Israel murdering their Brother, the Lord Jesus Christ, as a result, they were dispersed and scattered throughout all nations. And yet Israel is still marvelously preserved by God.

What happened to Israel is very similar to what happened to Cain.