What was the "mark of Cain"? The Torah only says God placed a sign on him so no one would kill him. Targum Pseudo-Jonathan on (Genesis 4:15) tells us what the sign was.
"The Lord sealed upon the face of Kain the mark of the Name great and honourable, that any one who might find him should not kill him when he saw it upon him."
The mark is a letter — or letters — of the divine Name itself. The Tetragrammaton, or a letter of it, was engraved on Cain's forehead. Anyone who saw him saw the Name of God and knew this was a person under divine protection, not a person to be hunted.
Why protect a murderer?
The Targum also records the promise: "Any one who killeth Kain, unto seven generations vengeance shall be taken of him." Cain, the first murderer, is given supernatural protection. Why?
Because the alternative — vigilante violence cascading through the new human population — would make Cain's sin the pattern of all human life. God, the Targumist suggests, halts the chain of killing. Cain's mark is not forgiveness. It is containment. The divine Name on his forehead says: this far, and no further. Seven generations of wandering, yes. But the cycle of vengeance stops here.
Jewish law will later teach that even a condemned criminal retains the image of God. The mark of Cain is the Targumist's earliest hint of that principle. The worst of sinners still bears the Name.