May you all have peace...
.... and the answer can be found at Genesis 6:1-4. Cain was afraid of being killed... and the only killers... or "fellers"... of men... at that time... were the Nephilim... "avengers of blood"... who were in the earth at that time. These, by reason of being spirit beings were "like God, knowing good (life) AND bad (death), for vengeance was their vocation.
These... were posted outside Eden... stationed in the Land of Fugitiveness... or "Nod." Once Adam, Cain and Seth began having other children, however, these began to notice "the daughters of men," and rather than "stand fast"... in their PROPER dwelling place... OUTSIDE Eden... the place where JAH had "set" them... they "forsook" that "station"... and went taking wives for themselves from among the daughters of men... from WITHIN Eden. (Adam and Eve were forbidden entry into the GARDEN that was IN Eden... and exiled to outside the garden... still within Eden.)
These Nephilim... "fellers"... were "mighty ones"... avengers of blood... whose responsibility was to kill those of man who committed "murder" IN Eden (a killing borne of jealousy and hatred, not self-defense (Gen. 4:23)), pursuant to the "eye for an eye... life for a life" rule), resulting in them being exiled OUT of Eden. Their job... was to avenge... on behalf of JAH. Thus, outside Eden was their "proper" dwelling place. But the daughters of men... were inside Eden... where such ones had to go in order to have relations.
Cain... was spared such vengeance, because he challenged JAH's mercy (just as Abraham, Jacob and Moses did), and it was granted him - Genesis 4:13-15.
I, myself, SJ, a slave of Christ, have shared it with you, just as it was given me (and another) by my Lord, JAHESHUA MISCHAJAH, the Son and Christ of the Holy One of Israel, whose name is JAH... of Armies.
I bid you peace.