Onan spilled it on the ground (so the Lord slew him too)

In his last killing, God killed Er for being wicked.

Er, Judah's firstborn, was wicked in the sight of the LORD; and the LORD slew him. Genesis 38:7

So Judah told Er's brother, Onan, to have sex with his dead brother's wife.

Judah said unto Onan, Go in unto thy brother's wife. 38:8

And Onan "went in unto" her all right, but in the process he "spilled it on the ground."

Onan knew that the seed should not be his; and it came to pass, when he went in unto his brother's wife, that he spilled it on the ground. 38:9

Then God, who was watching the whole thing, killed Onan for ejaculating outside the vagina of his dead brother's wife.

And the thing which he did displeased the LORD: wherefore he slew him also. 38:10

This story is used by believers to justify their condemnation of everything from masturbation (which is sometimes called "Onanism") to birth control.

But I think it's just another nasty, amoral Bible story. If there is a moral to the story, though, I guess it is this: Be careful where you ejaculate.

The Friendly Atheist - How many people did God kill in the Bible?

Onan