To be in more detail, the bible in chapter 4 verse 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.”
In the previous verse, Cain expressed his fear over God’s punishment. The murder of his brother would leave him able to be hurt by being killed himself, likely in revenge. God, who is about to kick out Cain from His presence, shows that He is still kind–and decided to stop people from looking (for) revenge. So He promises Cain that He will take revenge–times seven!–on anyone who kills Cain.
To seal the deal, and fight off all would-be attackers, God put a mark on Cain. We don’t know what this mark looked like, or if it was even visual. All we know is that it communicated loudly and clearly to all who met Cain that God would take revenge on anyone who killed the murderer of Abel.
Why would God do such a thing? Why not let Cain get what’s coming to him? Later, God will build into the Law procedures both for bringing justice on (people who commit crimes) and for helping murderers find safe place from those who would look (for) revenge. It seems God’s purpose here is focused on preventing the never-ending cycle of revenge to which humans are prone. For now, God simply insists on being the one to take revenge on terribly unfair treatment. In the New Testament, the Apostle Paul will show/tell about that this is still a role God demands to play today (Romans 12:19).

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