Why did God punish Cain and Onan so differently?

Question

Cain killed Abel in Genesis 4 and punished:

10 The Lord said, “What have you done? Listen! Your brother’s blood cries out to me from the ground. 11Now you are under a curse and driven from the ground, which opened its mouth to receive your brother’s blood from your hand. 12When you work the ground, it will no longer yield its crops for you. You will be a restless wanderer on the earth.”

Many centuries late, Moses commanded in Leviticus 24:17:

Anyone who takes the life of a human being is to be put to death.

Cain did not receive a death sentence for murder. On the other hand, Onan did for coitus interruptus in Genesis 38:

8 Then Judah said to Onan, “Sleep with your brother’s wife and fulfill your duty to her as a brother-in-law to raise up offspring for your brother.” 9But Onan knew that the child would not be his; so whenever he slept with his brother’s wife, he spilled his semen on the ground to keep from providing offspring for his brother. 10What he did was wicked in the Lord’s sight; so the Lord put him to death.

Deuteronomy 25:

7However, if a man does not want to marry his brother’s wife, she shall go to the elders at the town gate and say, “My husband’s brother refuses to carry on his brother’s name in Israel. He will not fulfill the duty of a brother-in-law to me.” 8Then the elders of his town shall summon him and talk to him. If he persists in saying, “I do not want to marry her,” 9his brother’s widow shall go up to him in the presence of the elders, take off one of his sandals, spit in his face and say, “This is what is done to the man who will not build up his brother’s family line.” 10That man’s line shall be known in Israel as The Family of the Unsandaled.

One would think that murder is more serious than onanism. Yet Onan was killed and not Cain. Why did God deal with these two wrongdoings so differently which were inconsistent with what he told Moses later?

Answer 1

"One would think that murder is more serious than onanism. Yet Onan was killed and not Cain."

OK, there are several things to unpack here.

1. Onan's crime

Onanism is just masturbation. Despite the name, that is not Onan's crime. His crime is that he slept with his dead brother's wife supposedly in order to provide an offspring, but he didn't want to provide an offspring he just wanted to sleep with her, so he had sex but pulled out, went back the next day, had sex but pulled out again, etc. That way he could keep sleeping with her but not actually give her an offspring that would not be his. Of course he could have just refused to be married to her, but Tamar was beautiful.

Technically, Onan was practicing birth control, but the whole point of why she (and the community at large) allowed him to marry (e.g. lawfully sleep with) her was to give her a child. In this way, he was committing an offense against his dead brother's wife but also against the community at large. Masturbation, in and of itself, was primarily an issue of cleanliness.

2. Why does God punish crimes and how is the severity reckoned.

In contrast with the God of the Bible, man has a warped view of crime and punishment, so things that don't seem serious to man are serious to God and vice versa. E.g. incorrectly saying a prayer in the holy of holies would result in instant death, but putting out someone's eye would not. Collecting sticks on the sabbath was a death penalty in Numbers yet slavery was not a crime at all. Thus human assessments of the magnitude of both offense and punishment are almost always way off the mark as they do not see into the future to fully understanding what is happening, nor can they peer into the world of spirit. The solution for those interested in exegesis is not to try to judge God by modern western standards of justice, but to try to understand what is happening from the point of view of the text itself and culture of the time:

One way to think about this is that Onan, by having sex with Tamar, was (according to custom) her husband, so if he was kept alive, then she would not be able to find another man with which to have a child. Onan's death allowed her to (ultimately) find someone who would not practice birth control, and from Tamar we get Christ. Thus God wanted to protect the line of Messiah and that line needed to keep going. Onan was trying to snuff that line out not only by refusing to continue the line himself, but as a result not letting anyone else continue it either[1].

Almost all of the God-commanded mass killings in the Bible, from Noah to the wiping out of the Caananite tribes, was to keep that line of Messiah pure and going until Christ was born. Thus Onan, by practicing birth control with Tamar, was a threat to the seed, that is the Promise, whereas Judah, by sleeping with a prostitute, was a friend of the Promise. Thus one was put to death and the other was honored, even though by human standards of justice Judah's crime was worse than Onan's. But God is interested in the seed, and his love is centered in his Son. Those who promote the promise prosper and those who oppose the promise are cut off. It does not matter what human justice is, because the mystery of the promise is foolishness to notions of human justice.

[1] Moses had not yet come along with the concept of certificates of divorce (which we are later told was only because of the people's sinfulness, and not according to God's heart for marriage), so there was not an option for Onan to say "sorry, I tried, now someone else can have a try" and to send her away after he got tired of her.

Answer 2

First, God did not ‘punish’ Cain. The ‘curse’ was brought on by by Cains unrighteous ‘act’.

GEN 4:11 Now you are under a curse and driven from the ground, which opened its mouth to receive your brother’s blood from your hand. 12 When you work the ground, it will no longer yield its crops for you. [snip]

The ‘earth’ was created by God. And by ‘soaking’ up unrighteousness (via blood) ‘it’ (the earth) responded. The ‘earth’ (or anything God creates) is not inanimate. The ‘whole of creation groans’ (Romans 8) because of this.

Second, God (that is, the Lord) did not ‘kill’ Onan. The Law did. The Israelites were clearly told there would be zero tolerance. (Exodus 23:21). The Law condemns, the Law ‘kills’. (2 CORINTHIANS 3:6).