The Wicked Witch was actually the nice witch that got turned into an evil one. See Flying Monkeys (Wizard of Oz).
Theodora was the nice witch that Oz met, who's sister tuned her evil by tricking her into eating the apple. Since Theodaro's sister knew she had more power, she kept on making her get evil. Since she had more power, she became more scarier and bloodcurdling.
Later, the wicked witch was killed, and the powerful one got turned back.