After history's first murder, God put a curse on Cain: "When you till the ground, it shall no longer yield its strength to you. A fugitive and a vagabond you shall be on the earth" (Genesis 4:12). Cain, instead of thanking God for not taking his life, complained that this punishment was "greater than I can bear!...[A]nd it will happen that anyone who finds me will kill me" (Genesis 4:13-14). God in His mercy responded by placing a mark on Cain "lest anyone finding him should kill him" (Genesis 4:15).
Speculations abound about this mark, ranging from the practical to the absurd. Among the practical suggestions is the idea that God gave Cain a distinguishing characteristic that would warn people not to harm him. Others believe that the "mark" was a sign to Cain alone of a promise of God's protection and not a warning to others at all.
The Hebrew word for "mark" is 'owth. Both meanings are possible—a warning to anyone who would harm Cain, as well as a banner or sign from God to Cain. The Bible does not clearly distinguish how we are to understand 'owth in this context. It is clear that God did not exact the death penalty that Cain deserved for murdering his brother Abel. However, the Bible does not say when or how Cain died.
Some believe that the wording of Genesis 4:23-24 implies that Lamech eventually murdered him. If that is true, whatever the mark of Cain was, God removed His protection. Perhaps He limited it to keeping someone from murdering Cain in revenge for the murder of Abel—but did not prevent anyone from killing him for some other reason.