Trasks vs. Hamiltons

John Steinbeck's East of Eden is divided into four individual parts, which I figure will be a good way to divide the posts. I have just finished the first part, and I have to say this book certainly brings your mind to places it doesn't usually wander to, unless you often think about sex, violence, disease, and family dysfunction.
In other words, it's pretty messed up and I still have 500 pages to go.
But it's dramatic. Everything that occurs is beyond inconceivable you just have to know what happens next, and in this way there's a lot to notice.
For example, even in the first part, there is a countless amount of symbols in regards to the fact that there is "only one story in mankind," the struggle between good and evil and man's ability to overcome temptation.
Some of these symbols are encased in geography, such as his intentional opening of the novel with a geography lesson about Salinas Valley, specifically mentioning the differences between the the Gabilan Mountains to the east, and the Santa Lucia Mountains to the west. He describes the Gabilans as "light gay mountains full of sun and loveliness," while the Santa Lucias are displayed to be "dark and brooding—unfriendly and dangerous," (3).
These comparisons continue into the storyline, as Steinbeck mentions two families, the Hamiltons and the Trasks. The Hamiltons are a large, loving, and moral family headed by Samuel Hamilton, an intelligent and respectable man who migrates to Salinas Valley from Ireland with his wife Liza to raise a family of four sons and five daughters, all very briefly elaborated on, but are portrayed to be healthy, God-fearing children who live comfortably together on the barren farm Samuel built their family home on.
The Trasks, however, are not candidates to be considered for any adjective used to describe the Hamiltons. The Trasks are a small family, headed by Cyrus Trask before his death, and his two sons, Adam and Charles. Cyrus Trask served in the Civil War, lost his leg, and contracted syphilis from a prostitute before being discharged, conveying him to be a man of relatively no morals. Later on in life, he ends up elevating his reputation in the eyes of others by claiming of his great advancements in the war when in reality, he did nothing to deserve praise or greatness, making himself a dishonest man as well as a vulgar one.
The Trasks are clearly the Santa Lucias to the Hamilton's Gabilans.
The Trask family is heavily elaborated on in comparison to the Hamilton family and I believe it is in order to highlight the importance of knowing the full story behind Adam and Charles' childhood together, specifically a story of their father's last birthday that he spent with his sons. Charles "took six bits and bought him a knife made in Germany" when Adam only "brought him a mongrel pup" (29-30), but it was clear Cyrus much preferred Adam's careless present to Charles' thoughtful one. The night Cyrus revealed to Adam that he "loved him better" (28) was also the night Charles severely beat and almost killed Adam, which makes the first direct connection to a biblical story, the story of Cain and Abel.
In this story, Cain and Abel, the sons of Adam and Eve, bring sacrifices to God. God ends up favoring Abel's sacrifice over Cain's and Cain murders his own brother out of jealousy. As punishment for his crime, God banishes Cain to Nod, the land that lies east of Eden (hmm..interesting).
Is there significance to the fact that Cain murdered his brother and Charles only attempted to? Is there a reason why Adam was not murdered as Abel was?