Genesis 38:10, The thing which he did. Onan refused to perform his levirate responsibilities likely because of greed. As second of three sons, he would have received only one fourth of Judah’s inheritance with the double portion going to the firstborn son. With the first born son dead, Onan stood to receive two thirds of the inheritance, since now he was the firstborn son. However, by raising up a son to take Er’s place, this would have given that son one-half of Judah’s inheritance and dropped Onan back down to one fourth of the inheritance again. Onan stood to gain financially by not performing his levirate responsibilities and rasing up a son to his older, deceased brother. Therefore, it appears that Onan spilled his seed on the ground because of greed, which is why YHVH killed him.