Firstly one ought to establish that according to Biblical understanding the earth was freshly created from day one. These events with Cain and Abel were decades after day one.
There might be speculations of a gap theory but it does not have strong support based off of one passage and one word ‘void’ which has been stretched to mean a pre-Adamic race and world. Bible speaks of two or three witnesses and this theory has only one.
The Hebrew כל denotes any or all and the LXX Greek πας also denotes any or all. The question then becomes who was Cain mostly concerned would find him and slay him?
List of potential slayers
Parents
Would his parents kill Cain and would Cain legitimately believe they would? This is unlikely. God in the Mosaic law demanded eye for an eye but here Cain is allowed to go free, it’s unlikely that Adam and Eve would have been expected to apply such judgment if God Himself doesn’t. A possibility but unlikely.
Siblings
He is the eldest, chances are he had sisters and evidently one was his wife G4:17. Would Cain be afraid of his siblings born or to be born? Unlikely again as it was Abel that Cain had a problem with and this due to God’s favor over Abel. Chances are he got along with the other siblings. Though this too is a possibility though unlikely.
Wildlife
God would give Cain a sign to protect him from being killed
“Then the Lord said to him, “Not so! If anyone kills Cain, vengeance shall be taken on him sevenfold.” And the Lord put a mark on Cain, lest any who found him should attack him.”
Genesis 4:15
The mark has to be self-evident and those who saw it had to know a seven-fold vengeance would follow.
Animals are not respecters of signs. If Cain had met a bear or a lion he would have had to fight it off. The sign was obviously for higher intelligent life and not animals. A much smaller and unlikely possibility.
Heavenly hosts on earth
This is the most likely fear Cain had. Cain probably was able to see Eden and it being guarded by an army of heavenly hosts
“He drove out the man, and at the east of the garden of Eden he placed the cherubim and a flaming sword that turned every way to guard the way to the tree of life.”
Genesis 3:24
But even if Cain did not see the cherubim (plural) he is aware of their existence.
Numerous times God would send an ‘angel’ to enact justice on behalf of the divine council.
In future occasions we read for example
“but David could not go before it to inquire of God, for he was afraid of the sword of the angel of the Lord.”
1 Chronicles 21:30
Or
“And that night the angel of the Lord went out and struck down 185,000 in the camp of the Assyrians. And when people arose early in the morning, behold, these were all dead bodies.”
2 Kings 19:35
There was an understanding that God could send an ‘angel’ to exact judgment. By giving Cain a sign from God Himself, no heavenly host would overlook that sign and exact death on Cain without also coming under severe judgment from God.
In Job for example the Accuser was allowed to exact judgment only within the confines established in the divine council meeting. But this did not prevent him from killing Job’s children and employees.
In conclusion
Another human seeing the sign Cain bore, could very easily have ignored the sign and slain Cain. Firstly he would have had to have known what the sign was. But Cain could also have had a good chance of defending himself against another human.
What Cain was most likely thinking was God’s judgment coming down in the form of a cherub with a flaming sword. Cain was satisfied with a sign knowing no heavenly host would override God’s commandment.
Worth noting is that Adam was banished to the east of Eden
“He drove out the man, and at the east of the garden of Eden he placed the cherubim and a flaming sword that turned every way to guard the way to the tree of life.”
Genesis 3:24
And Cain went to live East of Eden
“Then Cain went away from the presence of the Lord and settled in the land of Nod, east of Eden.”
Genesis 4:16
There’s no reason to believe that Cain was afraid of his own parents or siblings. Chances are he continued to interact with them even after Abel’s death. Text says Cain went away from the Lord’s presence not his own family’s presence. Text also says that God told Cain he would be a fugitive, Cain didn’t seem to want to become a fugitive, and being away from God’s face meant he was no longer under God’s protection. He wanted to be assured that he would be protected from being killed once he left God’s presence, (protected from God and His angels)
“When you work the ground, it shall no longer yield to you its strength. You shall be a fugitive and a wanderer on the earth.” Cain said to the Lord, “My punishment is greater than I can bear.”
Genesis 4:12-13
Eve clearly states that Cain killed Abel. Either God told Eve, or Cain did. Because if Cain never saw his parents again then they would have concluded they lost both sons.
“And Adam knew his wife again, and she bore a son and called his name Seth, for she said, “God has appointed for me another offspring instead of Abel, for Cain killed him.””
Genesis 4:25