Abel was the first human being to be killed by another person.
When Cain killed Abel, the Lord had not given mankind punishment guidelines for crimes which men commit against each other.
After Cain kills Abel, the Lord sets the punishment guidelines for one man killing another.
Genesis 4:15: “And the Lord said unto him, Therefore whosoever slayeth Cain, vengeance shall be taken on him sevenfold. And the Lord set a mark upon Cain, lest any finding him should kill him.”
The Lord meant that whoever killed Cain, both that person and others of his household (seven-fold of him) would be put to death. This law would apply to all mankind.
Years later, Lamech, a descendant of Cain, killed a man. Lamech told his two wives to hear his voice and harken unto his speech because he had killed a man. Therefore, if Cain be avenged sevenfold, truly he would be avenged seventy and seven fold.
Genesis 4:23-24: “And Lamech said unto his wives, Adah and Zillah, Hear my voice; ye wives of Lamech, hearken unto my speech: for I have slain a man to my wounding, and a young man to my hurt. If Cain shall be avenged sevenfold, truly Lamech seventy and sevenfold.”
Lamech was making his wives aware of the law for killing another person. They, along with him and others of his family would have to pay the penalty of death for his crime.
Lamech was the first man mentioned in the Bible to have two wives and the second man mentioned to have killed another person.
In Genesis 4:24, Lamech said, “If Cain shall be avenged sevenfold, truly Lamech seventy and sevenfold.”
Lamech meant that, after he had killed the man, the penalty for killing him, Lamech, or any other person would increase seventy and sevenfold.