You and I are not in the Bible, so obviously we don't exist, either. Now get real.
There is nothing in the Bible to indicate that Cain was Eve's first child. She said, "I have gotten a man from the Lord," not, "Wow, this is my first son."
The guilty fleeth when no man pursueth, or so the saying goes. Cain felt that people might want to get even with him or punish him for his sin. The Bible doesn't say that anyone tried to kill Cain. It only shows Cain whining about how unfair life is.