... the Wicked Witch of the West wasn't always evil? Elphaba (the birth name of the previously mentioned witch) had every intention of using her magic powers to make the world a better place- finding love for her sister (correctly guessed as the Wicked Witch of the East) and saving her love crush from a tragic end.
The result of her screw up was the Tin Man and the Scarecrow. Later frustrated that her attempts at creating her own fantasy miserably failed, Elphaba vowed a life of evil. Of course, there is way more to the story, but the moral here is that you can save yourself a lot of grief by letting the natural course of things unfold.