And last but definitely not least, we've got Cain-Abel action bursting the seams of this book. All of the other allegories revolve around this allegory. Why, you ask? As Samuel puts it,
"Two stories have haunted us and followed us from our beginning," Samuel said. "We carry them along with us like invisible tails—the story of original sin and the story of Cain and Abel." (22.4.15)
The tale of Cain and Abel isn't just about Cain and Abel: it's about people in general. Think of the Cain and Abel story as something that all of humanity has inherited: the curse of being jealous. Lee explains it this way:
"The greatest terror a child can have is that he is not loved, and rejection is the hell he fears. I think everyone in the world to a large or small extent has felt rejection. And with rejection comes anger, and with anger some kind of crime in revenge for the rejection and with the crime guilt—and there is the story of mankind." (22.4.64)
Yoda says pretty much the same exact thing in The Phantom Menace. It boils down to this: when we act out in hate or anger, it's usually out of some sort of fear. In this case, it's the fear of not being loved. Lee's explanation is actually a great explanation of literature in general: stories last because they continue to resonate with us, and who doesn't resonate with the fear of rejection?
But here's where it gets kind of tricky. Above we said that the whole Cain-Abel-jealousy thing is something that all of humanity inherited. Adam seems to subscribe to this idea when he says,
"What chance did we have? We are the children of our father. It means we aren't the first. It's an excuse, and there aren't enough excuses in the world." (22.4.40)
In other words, jealousy that may or may not lead to brother-murder is inevitable. Our parents felt it, their parents felt it, and everybody is going to keep on feeling it because that's what humans do.
But Lee is having none of this idea. For him, it all rests on the translation of timshel:
"But the Hebrew word, the word timshel—'Thou mayest'—that gives a choice. It might be the most important word in the world. That says the way is open. That throws it right back on a man. For if 'Thou mayest'—it is also true that 'Thou mayest not.'" (24.2.73)
So the anger and violence that comes from jealousy actually isn't inevitable at all. In the Bible, God doesn't ordain that Cain will be rejected and act out; he says that Cain has the option about how he will behave, and Cain chooses to murder his brother.
So let's apply this first to Charles and Adam, and then to Cal and Aron (also, you can take a look at Charles's and Cal's discussions in the "Characters" section to get more of the Cain angle). On the night Charles beats up Adam, he clearly has the intention of killing him with an axe. It comes to pass that he doesn't, and he doesn't even remember the event later, though he afterwards writes to Adam that "there's something not finished" (4.2.12). So then the action gets transferred on to the next generation; in other words, whether or not Cain kills Abel is going to rest on Cal and Aron.
Much later on, when Cal is still reeling from Adam's rejection of the money, Lee says to him, "You have a choice" (49.3.76). He means that whether or not Cal decides to do the mean thing rests with Cal and Cal alone—i.e. he can't blame it on Kate's genes or some inherent meanness in him. So even though Cal eventually does destroy his brother, the whole idea here is that we are trying to escape the vicious Cain-Abel cycle by acknowledging that, hey, it is a choice after all.