In 4:8, Cain kills his brother Abel. The Creator confronts Cain and sentences him to be "a vagrant and a wanderer on the earth" (4:10). Cain fears for his life and begs for mercy, and is given a distinguishing mark "so that none that meet him might kill him."
The descendents of the exiled Cain are named in short order in 4:17. We learn nothing about their lives until we get to Lemekh, whose words to his two wives are recorded in 4:23-24:
"I have slain a man for wounding me, and a lad for bruising me. For sevenfold is Cain avenged, and Lemekh seventy-seven."
The passage is cryptic and difficult to translate, and other interpretations exist. But I think the most straightforward - and also the most disturbing - is simply that he is boasting about his willingness to kill. "Perhaps, then, what Lamech is saying (quite barbarically) is that not only has he killed a man for wounding him, he has not hesitated to kill a mere boy for hurting him." (Alter)
Prager agrees: "Lamech boasts that if any man touches him, he will kill seventy-seven of his opponent's men in retaliation. This type of unbalanced retribution was the norm in all societies."
Notice the connection with Cain: man has taken God's mercy and perverted it into a literal license to kill. The result is society's descent into barbarism. Is it any wonder the Creator is angry?
CAIN'S SONS AND DAUGHTERS
The line of Cain disappear from the text, seemingly without trace. We do not know whether they intermarried with the descendents of Seth (chapter 5). What is interesting, though, is that Lemekh's wives Adah and Zillah are the first women mentioned by name after Eve, and Zillah and her daughter Na'amah are the first mother/daughter pair identified in the Bible. And there is a Rabbinic tradition that Na'amah was the wife of Noah.
THE NAMING OF NOAH
The text ties the name Noah - [noach] in Hebrew - to the verb [nachem], which incidentally has two meanings, both of which are in play here. Usually [nachem] means to comfort or to console, and that's how it is explained as relating to Noah's name. But also, and much less commonly, it can mean "to regret", and it is also used in that sense here, when the Creator is described as having "regretted" making man.
But the whole explanation is a little strange, beause the guy's name isn't [nachem], it's [noach]. The word supplied as an explanation is a near but imperfect match for the name it's supposed to explain. So where does [noach] come from? You guessed it - the name [noach] derives from our old friend, the verbal root [nachah] meaning rest and enjoyment.
So in fact, Noah's name not only promises comfort - it also hearkens back to Adam's restful state in the Garden of Eden, and before that, the Creator's rest on the first Sabbath.