The mark of Cain

[TonyChanYT]

New International Version, Genesis 4:15:

But the LORD said to him, “Not so; anyone who kills Cain will suffer vengeance seven times over.” Then the LORD put a mark [H266] on Cain so that no one who found him would kill him.

H266 occurs 79 times. Most frequently, it is translated as "sign", NASB:

So the LORD said to him, “Therefore whoever kills Cain, vengeance will be taken on him sevenfold.” And the LORD appointed a sign [H266] for Cain, lest anyone finding him should slay him.

It is the same Hebrew word in Genesis 9:

13 I have set my rainbow in the clouds, and it will be the sign [H266] of the covenant between me and the earth.

Gill:

and the Lord set a mark upon Cain; about which there is a variety of sentiments
(a): some say it was a horn in his forehead: others, a leprosy in his face; others, a wild ghastly look; others, a shaking and trembling in all his limbs; and others, that there was an earthquake wherever he stepped: and others will have it, that the dog which guarded Abel's flock was given him to accompany him in his travels, by which sign it might be known that he was not to be attacked, or to direct him from taking any dangerous road: some say it was a letter imprinted on his forehead, either taken out of the great and glorious name of God, as the Targum of Jonathan, or out of his own name, as Jarchi; others the mark or sign of the covenant of circumcision

The above are various possible physical interpretations of the mark.

(b): but as the word is often used for a sign or miracle, perhaps the better rendering and sense of the words may be, "and the Lord put", or "gave a sign"

i.e., God gave a promise to Cain

(c); that is, he wrought a miracle before him to assure him, that "whoever found him should not kill him": so that this was not a mark or sign to others, to direct or point out to them that they should not kill him, or to deter them from it; but was a sign or miracle confirming him in this, that no one should kill him; agreeably to which is the note of Aben Ezra,"it is right in my eyes that God made a sign (or wrought a miracle) for him, until he believed;'' by which he was assured that his life would be secure, go where he would; even that no one should "strike" him, as the word is, much less kill him.

In any case, God was gracious to Cain, even though he deserved to be killed. God gave Cain a chance to repent. Romans 2:

4 Or do you show contempt for the riches of his kindness, forbearance and patience, not realizing that God’s kindness is intended to lead you to repentance?

See also Why did God punish Cain and Onan so differently?.