I have 2 questions regarding the Book of Genesis:
- Why would God monologue to himself about the humans eating the fruit of life, if he isn't doing a monologue to himself, who is he talking about when he says "us";
- If Adam and Eve really are the first two people, when one of their sons kills the other and gets exiled, who is he talking about when he says "anyone who may see me may kill me"?
1) Notice in Genesis 1, that in the beginning, the Spirit of the Lord was hovering over the waters (that's the first mention of the Holy Spirit).
2) Later in the OT, when it mentions "the angel of the Lord", the person often first recognizes it as an angel, but then calls it "Lord". It's believed that this is because the "angel" was really "God the Son" (a.k.a., Jesus). Also about Jesus, John 1 tells us that Jesus was the one who created everything that exists.
3) The word for God in the Hebrew is plural, yet always used with a singular verb.
What all this suggests is that the writer of Genesis was already familiar with the Holy Trinity. So when God was speaking about creating in "our" image, it's believed that He was speaking to His 2 other parts.
Granted, some believe that God was speaking to the angels, but then, Paul tells us that we'll one day judge the angels...so we can't be made in their image, also.
In terms of Cain's exile:
The genealogies of Gen. 5 mention that Eve also had "many other sons and daughters".
It's also believed that the reason Cain would be afraid of the people in the other lands who might kill him is because, since they were all siblings, they'd also have known Abel. Plus, as you research the geographies of migration, you'll see that in the beginning, nobody really branched out too far. In other words, everybody was still pretty closely knit, and would've heard about the murder. So then after recognizing Cain's sin of killing Abel, these other siblings and/or their descendants may kill Cain for what he did to their brother/uncle, Abel.