Therefore; or, assuredly, as the word signifies, Jeremiah 2:32, Jeremiah 5:2, Malachi 11:17; that thou mayst see how I hate murder, and how impartially I shall punish all murderers; and that thou mayst be unhappily free from this fear, that thou mayst live for an example to mankind, for a terror to thyself and others. Sevenfold, i.e. abundantly; he shall be plagued with many and grievous punishments, as the phrase is used, Leviticus 26:28 Psalms 12:7, Psalms 79:12, and in many other places. A mark upon Cain. What this was, whether a trembling of his body, or a ghastliness of his countenance, or what other visible token of the Divine displeasure, God hath not revealed, nor doth it concern us to know.
Related Commentaries of Genesis 4:15
Genesis 4:15
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.