It is interesting to note that God did not subject Cain to capital punishment for which he richly deserved. Why is this the case? Is this an argument against capital punishment?

After The Flood

Capital punishment, it seems, was not instituted until after the Flood. God said:

For your own lifeblood I will surely require a reckoning: from every animal I will require it and from human beings, each one for the blood of another, I will require a reckoning for human life. Whoever sheds the blood of a human, by a human shall that person's blood be shed; for in His own image God made humankind. And you, be fruitful and multiply, abound on the earth and multiply in it (Genesis 9:5-7).

Still, Cain recognized that his crime was worthy of death. He said:

Today you are driving me from the land, and I will be hidden from your presence; I will be a restless wanderer on the earth, and whoever finds me will kill me (Genesis 4:14).

Summary

The fact that Cain was not put to death for killing Abel is not an argument against capital punishment. God had not yet instituted this procedure for those who took a life. Even though Cain was not put to death, he recognized that this would be a fair punishment for what he had done.