An early version of Doctor Strange in the Multiverse of Madness didn't make the Scarlet Witch a villain right away

Before the events of "Doctor Strange in the Multiverse of Madness", the Scarlet Witch - sickened by the death of her robot lover Vision (Paul Bettany) - had just bent reality and warped the mind of a entire small town by looking like a series of placid American television sitcoms. The results and fallout of his deformation were detailed in the television series "WandaVision". At some point between this series and "Multiverse," however, the Scarlet Witch came across the Darkhold, a forbidden, satanic-like spellbook of immense evil. Reading the book corrupted the Scarlet Witch, and she became perfectly willing to kill others and sacrifice lives in order to recreate her sitcom life for real. You know she's evil because of her heavy eyeshadow.

In "Assembled," Olsen admits she was surprised by the sudden decision to cast her character as a villain...and a scary villain at that. However, she also admits that playing villains is just plain fun, saying the ultimate power would indeed erase the traditional sense of ethics. So she was able to ride with it.

Waldron also pointed out that making Wanda a villain even makes sense. In the events of "Avengers: Infinity War", she deliberately killed Vision in order to prevent the genocidal monster Thanos from stealing a vital part of Vision's robotic makeup: a magic gem. His killing was undone by Thanos' new ability to rewind time, however, and his killing was for naught. Vision was killed by Thanos anyway. Maybe only one death from Vision's perspective, but Wanda seems to have recalled both. Thanos' temporal abilities had previously been given to him by Dr. Strange. We can see how the trauma of the event, facilitated by Doctor Strange, would cause her to crack.