NOTES for Gen 4:16-26
The fact that an entire civilization is built on the line of Cain says that the human race is poisoned from the beginning. The sacred writer shows how the terrible machine of evil gathers speed. The words, "if Cain is avenged sevenfold, then Lamech seventy times seven," mean precisely this. And Lamech's words, "I killed a man for wounding me," mean that the murder was committed in revenge for some evil done to him, Lamech. But in fact, is this not what we see throughout all of human history? As soon as a person gets some new technical achievement into his hands, the forces of evil soon take hold of it. This process is best described by the expression "arms race," which sounded out in the terrible twentieth century: that is the kind of arms race that has run throughout all human history.
But Seth was given in place of Abel: for wicked Cain, revenge, evil, and new deaths; for good Abel, a new birth, a new human being. This fact serves as an answer to superficial judgments that the Old Testament allegedly permits or even glorifies revenge: if that were so, revenge for Abel would have been glorified. But no, the birth of Seth is glorified for Abel.