Why did God punish Cain and Onan so differently?

[tonychanyt]

Cain murdered his brother. God punished him in Gen 4:

12 "When you work the ground, it will no longer yield its crops for you. You will be a restless wanderer on the earth.”

God did not kill Cain on the spot. On the other hand, Onan practiced coitus interruptus. God punished him in Genesis 38:

9 But Onan knew that the offspring would not be his. So whenever he went in to his brother’s wife he would waste the sperm on the ground, so as not to give offspring to his brother.

He was exploiting the situation for his own pleasure while denying Tamar the protection and security of children, which was her right under their laws and customs. He wanted the pleasure but refused the responsibility.

10 What he did was wicked in the Lord’s sight; so the Lord put him to death.

Why did God spare Cain but execute Onan?

Onan didn't know it, but the Messiah was supposed to come from Judah, his father. For refusing to cooperate in this matter, God killed him. God is sovereign. He did what he saw fit to do.

See also What was the sin of Onan?.