Why Did God Kill Onan?

Judah had 3 sons.  The first two were Er and Onan.  For easier reading I will say Er1 and Onan2, because that is the order in which they were born.

Er1 married Tamar and then died before they had children.  After his death, Er1's brother, Onan2, was left with several responsibilities that were a part of the custom of their day; customs that helped serve the interests of the dead brother and his widow.

1.  Onan2 was to take care of the widow Tamar - widows were extremely vulnerable in the old world.
2.  Onan2 was to have sex with Tamar so that she could bear a son.
     a.  Who would help take care of her (especially in her old age).
     b.  Who would carry on the name and lineage of the dead husband, which was very important in their culture.
3.  Onan2 was suppose to take care of the land and possessions that belonged to Er1 and eventually give it to Tamar's son (which was biolologically his, but belonged to his dead brother).
4.  Tamar's son that came as a result of Onan2 having sex with her would be considered Er1's son and carry on Er1's name, even though Onan2 would be the biological father.
5.  There may be other responsibilities that are not mentioned or implied in Genesis or elsewhere in the Bible.

When it came time to perform his sexual duty, Onan2 refused to impregnate his sister-in-law, so he practiced primitive birth control...he pulled out his penis before it was time to ejaculate.  The Bible does not tell us much about why Onan did not want a child:

But Onan was not willing to have a child who would not be his own heir. So whenever he had intercourse with his brother's wife, he spilled the semen on the ground. This prevented her from having a child who would belong to his brother (Genesis 38:9).

As a result of this action, God killed Onan2, because He considered his action evil.

WHY DID HE SPILL HIS SEED?

As a young Christian I heard a preacher say that God killed Onan because he was masturbating.  No kidding.  But this is not the case.  In reality, this story was designed to be a lesson for people to fulfill cultural obligations for the benifit of the deceased and of the widows.

Honor and carrying on one's name was huge in ancient Israel.  Keeping one's name alive after one's death was extremely important.  But if a man died before his wife became pregnant with a son, his name and his lineage was in danger of perishing.  And so, it became custom for the closest relative to impregnate the widow so that the name of the deceased would continue through the son's name.

However, if a brother2 had a son in the name of the deceased1, it carried certain risks for the living brother2, the biologogical father.   In another story (the book of Ruth), Ruth's nearest relative did not want to marry the widow (Ruth) because her nearest relative was afraid that the son born to them would jeapordize his own son's inheritance. 

In the Book of Ruth, the nearest relative knew that the son born to Ruth would not he his, but his relative's, so he refused to do his duty and impregnate Ruth.   He disassociated himself entirely from any son that could have been born to the widow, even though it would be his own biological son.

So there could be several reasons an older brother (or nearest relative) would not want to give the deceased a son:
1.  Anger issues - there are family issues that run deep in many families, and it could be that a brother would not want to give his dead brother a child and a name the continues.  "Let him die, and let his name die with him!"  Genesis has several examples of brothers who dislike, hate, or murder their brothers.
2.  Protecting his own son - In Ruth's case, the nearest relative did not want to jeapordize his own son's inheritance.  This is a bit mysterious, but it could be that if his first son died, then the inheritance and the land would go to the son that belonged to his brother's wife.  If this is the case, then his land would pass on to his brother's name.  Anybody else have suggestions?

WHY DID GOD KILL ONAN?

There were several episodes in the Bible where God killed somebody because they did something extremely evil.  Onan refused to take care of his brother, children laughed at a bald prophet and were malled by a bear, and Ananias and Saphira lied to the apostles in order to gain clout among the Christian community.  All of these people were killed by God, and their stories continued through generations in order to warn people in their times as well as in future generations.

In the case of Onan, the lesson is simply this: "Fulfil your family obligations, because God takes them very seriously."