Since we have some experts in the bible here, I would like someone to explain this..
9Then the Lord said to Cain, “Where is your brother Abel?” “I don’t know,” he replied. “Am I my brother’s guardian?” 10Then He said, “What have you done? Your brother’s blood cries out to Me from the ground! 11So now you are cursed [with alienation] from the ground that opened its mouth to receive your brother’s blood you have shed. 12If you work the land, it will never again give you its yield. You will be a restless wanderer on the earth.” 13But Cain answered the Lord, “My punishment is too great to bear! 14Since You are banishing me today from the soil, and I must hide myself from Your presence and become a restless wanderer on the earth, whoever finds me will kill me.” 15Then the Lord replied to him, “In that case, whoever kills Cain will suffer vengeance seven times over.” And He placed a mark on Cain so that whoever found him would not kill him. 16Then Cain went out from the Lord’s presence and lived in the land of Nod, east of Eden.
– Genesis 4:9-16 (HCSB)
Ok, there was Adam and Eve the first, they had two sons named Cain and Able. So that was four before one was slain, so what persons was Cain so afraid of.