After Cain killed Abel, whom did he have to fear?

TL;DR:

Cain was afraid of his own family members who were capable of seeking revenge.

Cain murdered his brother Abel, after God rejected Cain’s sacrifice but accepted Abel’s. As a result of the murder, God cursed Cain, making him “a fugitive and a wanderer on the earth” (Genesis 4:14). At this, Cain reveals his fear that, “whoever finds me will kill me.” We don't have to invent a pre-Adamic race to explain whom Cain feared after he killed Abel. It was simply his brothers and sisters and their families. Cain was right to fear revenge, but God’s hand still protected Cain from anyone who would avenge him.

from the old testament

Genesis 4 gives the account of Cain and Abel. God accepts Abel's sacrifice, but not Cain's. Out of anger, Cain murders Abel. God curses Cain with a difficult life and exile, and in Genesis 4:14, Cain expresses fear that someone else will be aware of his vulnerable position and kill him.
Genesis 4:1 is the first time a child of Adam and Eve's is mentioned. There is nothing that specifically says Cain was the firstborn. Likewise, there is nothing in the Bible that says Adam and Eve had no children between Abel and Seth. Cain could have had younger brothers and sisters, as well. With the near-perfect genetics of the second generation, their fertility would not be in question.
Genesis 5:4 says that Adam and Eve had many sons and daughters.
The ages of Cain and Abel are not given. All that can really be inferred is that they lived somewhat close to each other. Genesis 5:3 says that Adam was 130 years old when Seth was born. Seth was born sometime after Abel's murder. If 130 years started from the time Adam left Eden or if Adam and Eve ate the fruit shortly after they were created, and if Eve got pregnant with Seth immediately after Abel's death, Cain could have been as much as 128 years old when the murder occurred. Again, that is plenty of time for Adam and Eve's descendants to accumulate.
Despite being afraid after killing Abel, God protected Cain. Genesis 4:15 tells us: “‘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.”

from the new testament

Cain fearing others is not mentioned in the New Testament.

implications for today

Cain’s sin led to fear. He feared those who might try to avenge him. Sin always has more consequences than we can know. Yet, Cain should have feared God first. Had Cain listened to God when He warned Cain that sin was crouching at the door and that he must master it, Cain would not have murdered his brother and would not have had to deal with the consequences and secondary consequences, such as fear, that came as a result of his sin. Psalm 91:1 shows us that there is safety for those who trust in the Lord: “He who dwells in the shelter of the Most High will abide in the shadow of the Almighty. I will say to the LORD, ‘My refuge and my fortress, my God, in whom I trust.’” We, too, are called to live for God and to turn from sin. When sin crouches at our door, we can choose to fear God and do what is right instead of experiencing fear with the consequences that come when we do not follow Him.

understand

After murdering Abel, Cain feared relatives that would avenge him.
There were enough children born and enough time had passed that there would be plenty of family members for Cain to fear.
Despite his fear, God had His hand over Cain and protected him from anyone who would avenge him. God gave him other consequences for his sin.

reflect

How does guilt for our wrongdoings lead to fear?
The concept of consequences for sin yet God’s mercy is prevalent in Cain's story. What do you learn about God from this?
In what ways do I prioritize fear of worldly consequences over fear of displeasing God in my decision-making?

engage

Some people think that Cain being afraid proves that God made another race of people before Adam and Eve (because the text doesn't mention any other people, except Cain's wife in Genesis 4:17). However, this does not take into account that Adam and Eve had many other children, and that significant enough time had passed that the population had grown by the time Cain was afraid.
Cain's fear of retaliation led God to offer him protection. How does this narrative illustrate themes of mercy and forgiveness, even in the face of grave wrongdoing?
People tend to worry about being judged by man instead of by God, but the Bible tells us not to fear man but God who can destroy both soul and body in hell (Matthew 10:28).