Genesis isn't hard. From the accounts, we are all descended from Cain (although the account also mentions Cain complaining of his fear of being attacked by others, so that may not be accurate). Cain is not an altogether unsympathetic character, just the next generation of the Fallen. Prey to jealousy, murderous anger, self-pity and general
weininess, he was warned by God that the Fallen have the power of free will--"Thou mayest"--to triumph over evil.
He was given protection, not absolution, and exiled.
Maybe I am simple. Adam and Eve were alone, They had Cain who slew Abel. where did Cains brides come from? Were they his sisters? It gets confusing. It makes a nice story till logic appears. Maybe Paradise is West Va. and he did marry his sister. Apologies to West Va., but you guys elected Byrd. LOL