Genesis 38:10

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

The thing which he did was evil in the sight of Yahweh, and he killed him also.

And the thing which he did was evil in the sight of Jehovah: and he slew him also.

and that which he hath done is evil in the eyes of Jehovah, and He putteth him also to death.

What he did was wicked in the sight of the LORD, so He put Onan to death as well.