Tell me who else existed at the time Cain slew Abel, other than Cain, Abel, Adam and Eve. Please. There is NO record.
Short of showing where anyone is born between Abel and Enoch, you have NO choice but to admit the Bible is lacking in details and is NOT a historically accurate recording of what happened.
I don't have to tell you that since it is your assertion that no one else exists. He could have been Elmer Fudd, but that would make no difference. By your reckoning, nothing is a historically accurate recording of what happened, since they don't list all persons names.