Genesis 38:10 says, “What he did was wicked in the LORD's sight; so He put him to death also.” Onan's motivation was selfish; he used Tamar for his own pleasure, but refused to perform his legal duty of creating an heir for his deceased brother.
Ayon sa Genesis 38:10, "Ito'y kasuklam-suklam kay Yaweh kaya't pinatay rin siya." Ang layunin ni Onan ay sakim. Ginamit niya si Tamar para sa pansariling kaaliwan, subali"t tinanggihan niya na gampanan ang kanyang tungkulin alinsunod sa batas na kailangan niyang magbigay ng tagapagmana para sa kanyang namatay na kapatid.