The knowledge that whoever killed Cain would be avenged sevenfold, spread through the generations, so that when Lamech, the great-great-great grandson of Cain, killed a man, Lamech declared to his wives,
“If Cain shall be avenged sevenfold,
Then Lamech seventy-sevenfold” (Genesis 4:24).
Thus, Lamech promised to avenge himself against anyone who attempted to attack him even more than what God had promised would come to the attacker of Cain.
Lamech’s way of thinking shows that violence continued to spread on the earth, until finally, during Noah’s generation, God had enough of man’s violent ways, and destroyed the entire population of the world, except for one family of eight, with a global flood!
An impenitent murderer only increases his arrogance and promise of more violence.
This passages shows that ancient man understood numbers and rhetorical devices, for Lamech saw the completeness of seven, but he magnified that much more through the promise of self-vengeance of “seventy-sevenfold.” He knew how to use language and he knew how to use numbers, showing in an evil way, his pride and promise of retribution.
Contrast that with what the Lord Jesus taught us through the use of a similar use of numbers,
“Then Peter came to Him and said, ‘Lord, how often shall my brother sin against me, and I forgive him? Up to seven times?’ Jesus said to him, ‘I do not say to you, up to seven times, but up to seventy times seven'” (Matthew 18:21-22).
Someone has hurt you. What shall you do? Shall you avenge yourself seventy-sevenfold or forgive the offender seventy times seven?