There's a reason we call Elphaba the John Locke (Lost version) of Oz. Like Locke, her life seems to be one big string of failures and disappointments. This is a fact that Elphaba herself notices with a lot of pain. She never seems to achieve her goals, and her failures range from minor, such as Chistery not progressing in his speech, to catastrophic, such as Fiyero's death. The latter is arguably not Elphaba's fault, but she thinks it is, which may be more important.
As the book progresses, Elphaba grows increasingly defeatist and stops trying. This attitude may be a form of evil itself, if evil can be defined as an "absence" of emotion or feeling. The psychological effects of defeat and loss are hugely important for Elphaba's character. It's no wonder she loses it when confronted with her freakishly effective doppelganger Dorothy.