Cain's response to the curse, in Targum Pseudo-Jonathan on (Genesis 4:14), includes a nuance the Hebrew does not spell out. "Behold, Thou hast cast me forth today from the face of the earth, and from before Thee is it possible to be hidden? And because I am a wanderer and an exile in the earth, any just one who findeth me will kill me."

The Hebrew says "anyone who finds me." The Targumist sharpens it: any just one. Cain fears not random violence but the moral response of the righteous. The people most likely to kill him are the ones most capable of recognizing what he has done. The tzaddikim are the threat, because they uphold the very justice he has broken.

This is a remarkable window into Cain's mind. He has just denied there is a Judge, but he is now terrified that the righteous will become judges themselves. The moral universe he tried to deny has caught up with him.