Adam's descendants, through Cain, are Enoch, Irad, Mehujael, Methusael, Lamech and Lamech's children (Genesis 4). His descendants through Seth, however, are Enos (Enosh), Cainan (Kenan), Mahalealel, Jared, Enoch, Methuselah, Lamech, Noah and Noah's sons (Genesis 5). Only six generations are recorded coming out of Cain while nine are listed through Seth.
We need to do a little Biblical detective work to determine what happened to Cain's descendants and the fate they suffered.
An Evil Inheritance
Cain first revealed a rebellious attitude toward God by not offering an acceptable sacrifice like his brother Abel. He then refused to acknowledge his error when he was confronted (Genesis 4:4 - 7). His ultimate "solution" regarding his relationship with God was to violently murder his brother who had done him no harm (see 1John 3:11 - 12 and Jude 1:10 - 11)!
Cain's attitude of refusing to obey his Creator and willingness to use violence to achieve his goals was passed down to his descendants.
"They (the posterity of Cain) were intolerable in war, and vehement in robberies; and if any one were slow to murder people, yet was he bold in his profligate behavior, in acting unjustly, and doing injuries for gain." (Antiquities of the Jews by Josephus, Book 1, Chapter 2, Section 2).
Wicked Seventh Generation
The Bible offers us evidence that Cain's descendants did indeed continue his violent and self-centered lifestyle. The words of Lamech, spoken before the flood, are considered the oldest recorded poem. Representing the seventh generation of humans on earth through Adam and Cain, he reveals himself to be a vain, vengeful man.
Lamech said to his wives, "Adah and Zillah, listen to me; wives of Lamech, hear my words. I have killed a man for wounding me, a young man for injuring me. If Cain is avenged seven times, then Lamech seventy-seven times." (Genesis 4:23 - 24, NIV).
While a few Biblical commentaries (JFB, Adam Clarke's) believe Lamech killed in self-defense, the majority (Holman, Matt Henry, Eerdmans, Biblical Illustrator, etc.) teach his words show he was a boastful, violent man full of resentment.
Lamech's demand that he receive even greater leniency for his sins than Cain got from God highlights his perverse attitude. One popular Bible commentary interpreted Lamech's words as the following.
"The idea is this: whoever inflicts a wound or stripe on me, whether man or youth, I will put to death; and for every injury done to my person, I will take ten times more vengeance than that with which God promised to avenge the murder of my ancestor Cain." (Keil and Delitzsch Commentary on the Old Testament, see also The Biblical Illustrator for Genesis 4:23).
Tools of Violence
Tubal-Cain, whose name means "thou will be brought of Cain" (Strong's Concordance #H8423), was one of Lamech's sons. He was the first person to work with bronze and iron (Genesis 4:22). He not only made domestic utensils out of metal but also implements of war. His weapons were used to add further fuel to humanity's indulgence in pre-flood violence.
"The first smith on record, who taught how to make warlike instruments and domestic utensils out of brass and iron." (Adam Clarke's Commentary on the Bible, comments on Genesis 4:22).
"an instructor of every artificer in brass and iron; he taught men the way of melting metals, and of making armor and weapons of war, and other instruments, for various uses, out of them . . ." (John Gill's Exposition of the Entire Bible, Genesis 4:22).
Enough is Enough
By the time Noah is 480 years old God has had enough of the descendants of Cain, and the rest of humanity's, willingness to constantly pursue evil.
And the Lord saw that the wickedness of man was great on the earth, and every imagination of the thoughts of his heart was only evil continually. And the Lord repented that He had made man on the earth, and He was grieved in His heart . . . Now the earth also was corrupt before God, and the earth was filled with violence (Genesis 6:5 - 6, 11, HBFV).
The Lord then decides to cleanse the earth of its wickedness through a flood. He commands his servant Noah to begin building an ark (Genesis 6:13 - 22).
Why Noah?
God, in spite of his words in Genesis 6, clearly wanted humanity to continue on earth. He therefore decided to show grace (favor) to Noah who, though certainly not sinless, worshipped and obeyed Him with his whole heart.
But Noah found grace in the eyes of the Lord. These are the generations of Noah. Noah was a righteous man and perfect in his generations, for Noah walked with God (Genesis 6:8 - 9, HBFV).
Noah "walked with" or had faith in the Lord just like his great grandfather Enoch (Genesis 5:22). They were part of a long line of descendants, beginning with Seth's son Enos, who acknowledged and worshipped the true God.
And there was also a son born to Seth, and he called his name Enos. At that time men began to call upon (call themselves after or proclaim) the name of the Lord (Genesis 4:26, HBFV).
Disappearing Descendants
Cain and his descendants, though quite industrious and creative (Genesis 4:17, 20 - 22), were many of the chief promoters of wickedness in the pre-flood world.
The descendants of Adam's surviving sons other than Cain and Seth (possibly thirty of them) also played a role in promulgating evil. Even the vast majority of those in Seth's line seemed to have indulged in disobedience (see Genesis 6:1 - 4).
Noah and his three sons, along with their wives, were saved in the ark when the floodwaters covered the earth. No one else, in spite of Noah's 120 years of preaching repentance and righteous living (1Peter 3:20, 2Peter 2:5), were willing to abandon their evil ways.
Lamech's children were likely, though not directly stated, the last generation of Cain's descendants produced before the flood. They, along with at least three billion others living on earth, perished in the flood due to their sins.