Why does the Torah continue to relate the story of Kayin following his punishment?
Ramban #1: To teach us that Hashem is "Erech Apayim", inasmuch as, in spite of his sin, Kayin had (and saw) children and grandchildren. 1
Ramban #2 (citing a Midrash): To teach us that he lived many years, right up to the Flood, 2 when he, together with all his descendants died. He did not go in peace down to the grave.
Of what significance is Kayin's genealogy, seeing as all of his descendants died in the Flood?
Ramban: To teach us that they were the world's first builders and inventors - of cities, shepherding, musical instruments and metal-making. 1
Why does the Torah record the episode of Lemech and his two wives?
Rashi: The Torah wants to tell us that the death of Hevel would be avenged after seven generations; that Lemech and his son Tuval Kayin (the seventh generation after Kayin) did indeed kill Kayin. 1
Ramban: To teach us that, although Lemech had children, they were wiped out before they had a chance to reproduce.
What is the significance of Lemech's two wives, and of the names "Adah" and 'Tzilah"?
Rashi: This was the practice of the generation of the Dor ha'Mabul. One wife was for Piryah ve'Rivyah - 'Adah' - because (physically) she disgusted him (Onkelos translates 'Surah' (rejected) as 'Adah'; 1 the other, for pleasure (intimacy) - 'Tzilah' (since she was constantly to be found 'in his shadow.'
Yerushalmi Yevamos, 6:5: See answer #1; 'Adah' (a Lashon of 'Idun', pleasure) - because he derived pleasure from her body; 'Tzilah' - because he sat in the shade of her children.
See Targum to Bereishis 49:10 .
QUESTIONS ON RASHI
Rashi writes: "This [account] comes to teach us how Hashem fulfilled His word... Lemech arose and killed Kayin." But later (Rashi to 4:24), Rashi cites another Midrash that Lemech did not kill anyone! If so, what does this account teach us?
Gur Aryeh #1: Lemech's wives wished to separate from him (Rashi to 4:23); this shows their great fear of Kayin's punishment (even many generations later).
Gur Aryeh #2: We learn from here that all of Kayin's progeny was cursed to be wiped out, to the extent that they feared having any more children. [We might have thought that only Kayin himself was punished, and not his offspring.]
Rashi writes: "'Two wives' - This was the practice of the generation prior to the flood ..." How does Rashi derive this?
Gur Aryeh: The verse emphasizes "two wives;" they would marry a pair of wives, to serve two different functions.
Rashi writes: "... He would administer a drink to make her sterile...." If so, how did Tzilah conceive and have children?
Gur Aryeh: In this case, the drink was ineffective, and she conceived. This is why the verse adds, "And also Tzilah gave birth..." (4:22).
