was evil in the sight of Yahweh

The phrase "in the sight" refers to Yahweh seeing Onan's wickedness. AT: "was evil and Yahweh saw it" (See: rc://en/ta/man/translate/figs-idiom)

Yahweh killed him also

Yahweh killed him because what he did was evil. This can be made clear. AT: "So Yahweh killed him also" (See: rc://en/ta/man/translate/figs-explicit)