Let’s distinguish between her BEING wicked, in her own mind, and her being painted as wicked.
The latter occurs after she refuses to ally herself with the Wizard (and, in effect, Madame Morrible) when she discovers what they’re up to – the last point at which she might have been convinced to do otherwise (“Wonderful”) ends when she sees Dr. Dillamond in the shadows, now just a mute animal.
But she’s convinced she’s doing the right thing, against them (“Defying Gravity”). It’s only after Madame Morrible has used her Press Secretary position to paint such a picture of Elpheba and she’s had the …
… misfortunes of Boq (partly her fault, if only by giving Nessa access to the Grimmercy) and Fieyro (sacrificed to save her, and turned by her spell to save his life) and, indeed, Nessa herself, whose death was a pawn’s sacrifice by the Wizard, Madame M., and unknowingly Glinda.
It’s only AFTER all that that she’s WICKED, in the sense that she’s saying, “To hell with trying to do good.”
- Rick