God kills another son of Judah

Genesis 38:8 says, “And Judah said unto Onan, Go in unto thy brother's wife, and marry her, and raise up seed to thy brother.

38:9 And 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, lest that he should give seed to his brother.

38:10 And the thing which he did displeased the LORD: wherefore he slew him also.”  I don’t understand this.  Er is dead and Onan’s father tells him to marry his brother’s wife and get her pregnant.  This is weird and gross but I guess it isn’t so bad.  Why does Onan know that the seed should not be his, but yet be killed by god because it didn’t please him?  If Onan spilled his seed on the ground than he was having sex with her in the first place why didn’t god just make one of his seeds get up there before he was done.  Lots of children are born because coitus interruptus didn’t work.  Is there really a need for god to kill him?