What is the theme in this book (not a one word theme such as "greed") ?

One main theme of the novel is the battle between good and evil. Juana and Kino represent good.... those who wish to rob them represent evil. You might even consider the members of the community, who turn their backs in jealousy evil. Kino's pearl changes their lives, but not for the better. Jealousy, greed, and desire surround the small family.... and they're left to battle on their own. In the end, the evil the pearl has created defeats the good of the family. The family is unable to withstand alone.... the family is destroyed as a result.