וַיֹּ֧אמֶר ל֣וֹ יְהוָ֗ה לָכֵן֙ כָּל־הֹרֵ֣ג קַ֔יִן שִׁבְעָתַ֖יִם יֻקָּ֑ם וַיָּ֨שֶׂם יְהוָ֤ה לְקַ֙יִן֙ א֔וֹת לְבִלְתִּ֥י הַכּוֹת־אֹת֖וֹ כָּל־מֹצְאֽוֹ׃
여호와께서 그에게 이르시되 그렇지 않다 가인을 죽이는 자는 벌을 칠 배나 받으리라 하시고 가인에게 표를 주사 만나는 누구에게든지 죽임을 면케 하시니라
Rashi
לכן כל הרג קין VERILY, WHOSOEVER SLAYETH CAIN — This is one of the verses where the construction is elliptical (literally, which abbreviate their statements), which give only a suggestion and do not fully explain. “Verily, whosoever slayeth Cain” express a threat, suggesting the consequence — “thus shall be done to him”, “thus and thus shall be his punishment”, without explicitly explaining what the punishment would be.
Chizkuni
לכן כל הורג קין, “therefore, because Kayin has already been so humbled,”
Da'at Zekenim
לכן כל הורג קין שבעתים יוקם, “therefore whosoever slays Kayin, vengeance will be taken on him sevenfold.” The plain meaning of this verse is that seeing that Kayin was worried about being murdered, G–d assured him that anyone murdering Kayin even after he had lived seven generations would be held accountable by Him. This is why Lemech who held himself as guilty of killing Kayin, even though he did so unintentionally, told his wives what would happen to anyone killing him on that account (Compare verse 25 in this chapter.) Lemech’s children were the seventh generation, after Adam. (Adam-Kayin-Chanoch-Irod-M’chuyael-M’tushael-Lemech-(Yaval,Yuval Tuval Kayin) He told his wives that the whole generation of such a killer fathered by himself would be held responsible for his murder by G–d, seeing that what he had done was due to his being blind and therefore totally innocent of any intent to harm a human being.