Rashi on Leviticus 20:17/רש"י על ויקרא כ׳:י״ז

1חסד הוא IT IS A WICKED THING — It is an Aramaic expression, the Hebrew חרפה (disgrace) being in that language חסודא (cf. Onkelos on Genesis 34:14). A Midrashic explanation of it (of חסד הוא) is; If you should say, "But Cain married his sister!" then I reply, Cain's case was an exceptional one; an act of kindness (חסד) was done by the Omnipresent in order that His world might be built up through him (i. e. He made the propagation of the human race possible through this union), as it is said (Psalms 89:3) "The world was built up through חסד, loving-kindness" (Sifra, Kedoshim, Chapter 11 11; Sanhedrin 58b).