Genesis 4:24

If Cain shall be avenged sevenfold, truly Lamech seventy and sevenfold.

1. Lamech was afraid of the consequences of his murder, just as Cain had been afraid.
2. Lamech was familiar with Cain's story and concerns.
3. Murder was apparently uncommon. This was, perhaps, the first murder since Cain killed Abel.
4. Lamech felt, for some reason, that his murder would bring 10 times more revenge upon him than Cain's murder of Abel.