And the thing which he did displeased the LORD. Less neatly the Jews speak about this matter. I will contend myself with briefly mentioning this, as far as the sense of shame allows to discuss it. It is a horrible thing to pour out seed besides the intercourse of man and woman. Deliberately avoiding the intercourse, so that the seed drops on the ground, is double horrible. For this means that one quenches the hope of his family, and kills the son, which could be expected, before he is born. This wickedness is now as severely as is possible condemned by the Spirit, through Moses, that Onan, as it were, through a violent and untimely birth, tore away the seed of his brother out the womb, and as cruel as shamefully has thrown on the earth. Moreover he thus has, as much as was in his power, tried to destroy a part of the human race. When a woman in some way drives away the seed out the womb, through aids, then this is rightly seen as an unforgivable crime. Onan was guilty of a similar crime, by defiling the earth with his seed, so that Tamar would not receive a future inheritor.

Wherefore he slew him also - The sin of Onan has generally been supposed to be self-pollution; but this is certainly a mistake; his crime was his refusal to raise up seed to his brother, and rather than do it, by the act mentioned above, he rendered himself incapable of it. We find from this history that long be fore the Mosaic law it was an established custom, probably founded on a Divine precept, that if a man died childless his brother was to take his wife, and the children produced by this second marriage were considered as the children of the first husband, and in consequence inherited his possessions.

And the thing which he did displeased the Lord,.... Being done out of envy to his brother, and through want of affection to the memory of his name; and it may be out of covetousness to get his estate into his own hands, and especially as it frustrated the end of such an usage of marrying a brother's wife; which appears to be according to the will of God, since it afterwards became a known law of his; and it was the more displeasing, as it was not only a check upon the multiplication of Abraham's seed as promised, but since the Messiah was to come from Judah. This was doing all to hinder it that lay in his power:
wherefore he slew him also; in like manner as he had slain his brother, Genesis 38:7.