Boyd’s Bible Dictionary:

(death). Punished with death (Ex. 21:12; Num. 35:30-31); but cities of refuge provided for the escape of the involuntary slayer (Ex. 21:13; Num. 35:32; Deut. 19:1-13).

Concise Bible Dictionary:

The first death was caused by murder when Cain slew his brother Abel, and the second recorded is when Lamech said, “I have slain a man to my wounding,” or “for my wound”; which may mean that he did it in self-defense (Gen. 4:23). God set a mark upon Cain that none should kill him; and Lamech said, “If Cain shall be avenged sevenfold, truly Lamech seventy and sevenfold.” After the flood God made a definite law concerning murder. God would require expiation for the blood of man, whether it was shed by beast or by man; at the hand of every man’s brother, or kinsman, God would require the life of man. “Whoso sheddeth man’s blood, by man shall his blood be shed: for in the image of God made He man” (Gen. 9:5-6). This injunction was repeated in the law, and has never been rescinded or modified. Neither does the New Testament in any way alter it: indeed it incidentally confirms it by declaring that the magistrate does not bear the sword in vain (Rom. 13:4). God claims the life of man, and none can set aside His rights.