Hemingway's novel takes up the idea of good and evil directly in one passage concerning David's story about his father. Here, David's father is depicted as denying evil and denying its power. David does not possess the same ability to overcome evil by dint of denial, though he tries. Ultimately, evil is associated with an inevitable loss of innocence, a loss that mirrors that of the biblical figures Adam and Eve.
There is a symbolic echo of his father's denial of evil in David's deteriorating relationship to Catherine. On only one occasion, David comments that the changes in Catherine are a form of perversion. For the rest of the novel, David attempts to understand, excuse and empathize with the changes in Catherine as she experiences a crisis of identity.
In the end, David accepts his loss of Catherine as he had learned to accept the loss of his innocence on his trek in Africa.
Hemingway stated that the book is about "the happiness of the Garden that a man must lose."
The "fall" from good to evil, so to speak, can be seen in the loss of happiness that David possesses at the opening of the novel. This simple and uplifting happiness is present also when David discovers the elephant in the moonlight.
This is fundamentally a narrative of a boy's loss of innocence, as David betrays the elephant, enabling the hunters to close in for the kill.
The loss of happiness harkens back to Romantic notions of good and evil as equivalents to innocence and experience (see William Blake). Similarly, the biblical story of Adam and Eve is often understood along these lines. The introduction of experience is also the introduction of evil into the Garden of Eden.
Certainly, for David the loss of innocence can be seen in the two examples of the deterioration of his idyllic moments, with Catherine and with the elephant. Evil, in his moments, is related to a separation from the "good" and "pure". Evil is not a force of malice in the novel, with the exception of Catherine's act of burning David's stories.
This act is one of malice and functions as the final division between David and Catherine. This act also definitively severs David and Catherine from the simplicity and love that characterized their relationship at the beginning of the novel.
There is no more denying that things have changed, gone bad, and that, as people, David and Catherine have lost the qualities that once helped keep them together. Those qualities are definitively "good" while those which develop to break the couple apart are evil.
Catherine's identity crisis is, perhaps signficantly, a crisis of knowledge. Much like the biblical story of Adam and Eve, it is a crisis of knowledge that removes the couple from innocence.