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.”

On the other hand, Onan practiced coitus interruptus. God punished him in Genesis 38:

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.

 

[Jim C]

I believe it all boils down to what God says in Exodus: "I will show mercy on whom I will show mercy"
Besides, at that moment Cain was the only other man on earth besides his father; while with Onan, God had other options available.

 

[Wick Stick]

On the other hand, Onan practiced coitus interruptus. God punished him in Genesis 38:

Why did God spare Cain but execute Onan?

Onan was meant to provide an heir for his deceased brother by impregnating his sister-in-law. By intentionally not getting her pregnant, he was essentially ending his brother's clan permanently. And also providing a reason to go on having sex with the sister-in-law. This is rape/genocide... a bit more serious than coitus interruptus.