Or HaChaim on Genesis 4:15/אור החיים על בראשית ד׳:ט״ו

1 לכן כל הורג קין שבעתים יוקם. This is why he who murders Cain will be avenged sevenfold (or after seven generations). The reason the Torah wrote לכן, therefore, is to show that Cain knew full well that if G'd would not protect him, anyone who encountered him (Cain the murderer) would kill him. G'd placed a sign on Cain's forehead as a sign of pity because Cain had finally shown awareness of G'd's השגחה, supervision of the fate of every individual. This "sign" was the same as the tradition that by merely looking at a צדיק one can tell that he is a G'd-fearing person (compare Exodus 12,13: "this blood will be a sign for you." This means that the מצוה you have performed in preparing the paschal lamb will be accounted as a sign.) Such "signs" act as protection for the righteous against a variety of dangers.