Cain: Which people did he fear?
The Bible tells us that
Adam and Eve had three sons: Cain, Abel and Seth. We read in Genesis 1:28: "and God blessed them (Adam and Eve); and God said unto them, Be fruitful, and multiply and replenish the earth". Adam and Eve had other children - read Genesis 5:4.
So, by the time Cain received the punishment from God to be a vagabond on earth, there could have been many descendents of Adam
and Eve who wished to revenge the blood of Abel.
Further speculation on the subject like the assumption that God created other people except Adam and Eve, has no scriptural foundation at all.
All this is just speculation, and not important at all. Cain and all
his descendants died a few years later (Genesis 7). Jesus said we must not fear people who can only kill the body (Matthew 10:28). We should rather fear God who can destroy both soul and body in hell.