Since John Steinbeck's novel East of Eden is a massive book, it's difficult to come up with any general statements that incorporate its entire meaning. However, the novel is a retelling of parts of the Book of Genesis, which chronicles the downfall of the human race and is the birthplace for some of the most basic elements of good and evil in the Western canon, so it's worth thinking about Steinbeck's relation to this question. Again, while the sheer scope of the novel makes it difficult to make sweeping, generalized statements about any one theme, I think it's safe to say that Steinbeck questions traditional, black and white evaluations of good and evil through his construction of the characters Cal and Aron.
In the novel, Aron roughly corresponds to Abel, and Cal roughly corresponds to Cain. This relationship is pretty easy to grasp from the get-go, so the reader already expects that Aron is going to die, most likely because he is betrayed by Cal. Abel, after all, is the righteous son of Adam who wins favor from God, while Cain is the outcast who murders Abel. In the Bible, this story is fairly straightforward, and it's pretty easy to tell right from wrong: Abel has God's favor so he is good, while Cain does not have God's favor and is therefore evil.
However, through the course of the novel, most readers find themselves sympathizing with Cal far more than Aron. Cal is a hardworking, quiet, introspective boy who wants nothing more than to earn his father's affection. Aron, on the other hand, is a smarmy, self-absorbed, downright annoying individual. While he certainly doesn't deserve to die in France, we also understand why Aron is "betrayed" by Cal, and few readers would utterly condemn Cal for doing so.
By constructing nuanced characters who are neither wholly good nor wholly evil, Steinbeck questions the overly simplistic dichotomy of good and evil offered in Genesis. While his theory of good and evil is no doubt more complicated than this answer allows for, it remains safe to assume that, on the whole, Steinbeck uses the complex characters of Aron and Cal to subvert the overly simplistic, black and white presentations of good and evil as they are found in the Bible.