Who was Cain afraid of?

Cain murdered his brother Abel and as punishment was forced to wander the Earth. In Genesis 4 we read,

And the LORD said, "What have you done? The voice of your brother's blood is crying to me from the ground. And now you are cursed from the ground, which has opened its mouth to receive your brother's blood from your hand. 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. Behold, you have driven me today away from the ground, and from your face I shall be hidden. I shall be a fugitive and a wanderer on the earth, and whoever finds me will kill me." 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. Then Cain went away from the presence of the LORD and settled in the land of Nod, east of Eden.

Cain knew his wife, and she conceived and bore Enoch. When he built a city, he called the name of the city after the name of his son, Enoch. (Genesis 4:10-17)

God's punishment meant that Cain was to wander the Earth and the inability for him to grow crops presumably was to help enforce this. The phrase "fugitive and a wanderer" (ESV) or "homeless wanderer" (NET) does not necessarily imply other people, other than his parents, existed at the time but the comment "whoever finds me will kill me" suggests there were other men. Later Cain builds a city which presumes several people lived there.

This has been raised as a difficulty, along with Cain's wife, if all men descend from Adam and Eve and Cain was their first son. Genesis does not give an exhaustive history of the world but there is minimal difficulty if one pays attention to what is told us.

Adam and Eve had many more children, we are told of 3 but given their perfection and longevity one would presume they had very many children. And Genesis informs us of this,

The days of Adam after he fathered Seth were 800 years; and he had other sons and daughters. (Genesis 5:4)

Birthrates for fertile women are every 2 to 4 years depending on breastfeeding method. Given that Cain was likely conceived shortly after the Fall which probably within several days of creation and assuming a gap of 2–4 years and no multiple births Eve would have had 26–51 children by age 100. I do not know the average age of menopause of women who live close to 1000 years though 500 would not be unreasonable. Sarah lived to 137 and was post-menopausal sometime before 90.

These children would have had further children, even with a low annual growth rate of 5% starting from 2 people we get ~250 people within 100 years, ~1100 people by 130 years and ~35000 by 200 years though the growth rate was possibly much higher at that time in history.

Cain married a sister as many of Adam's sons would have. Even as late as Abram born 2000 years after creation we have a man marrying a half-sister.

A key to understanding Cain's comment about men wanting to kill him comes from Seth's birth. I think it unlikely Seth was Adam's third son.

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)

This passage shows us that Eve viewed Seth as replacing Abel, but Seth was born when Adam was 130:

When Adam had lived 130 years, he fathered a son in his own likeness, after his image, and named him Seth. (Genesis 5:3)

So the murder of Abel could have been as late as 130 years after creation with a conservative number of people being 1100. Some of whom Cain could have seen as a threat to his life. And even if Abel's murder was many years prior while many of the people were younger and the population fewer, the possibility of future attacks on Cain may have been on his mind.