How much flashback time do you want in a movie that's already two hours long, that needs to also show where Strange is at right now, and give at least some time to America Chavez?
You could probably do it in a minute or two. Like, when Strange goes to her orchard, and she says she's being reasonable, and Strange says no, she could try to just mind-whammy him then and there. Maybe he manages to fight free of some mindscape weirdness by casting a spell to conjure forth someone's greatest fear, and that's when we get to see a cascade of images from Wandavision, then a montage of her being corrupted by the Darkhold.
The trick for me, though, is that I don't want Wanda's agency taken away. I want her villainy to be the result of an intentional character choice, not an unexpected consequence or a situation where she's being victimized by an inanimate object she cannot later defeat. She's been manipulated by Strucker, Ultron, and Agatha, and she's lost loved ones to Thanos and even that S.W.O.R.D. guy whose name I forget.
It just frikkin sucks if she becomes a villain because she's gone cuckoo or because an evil book makes her do it.
Heck, going full nihilist would work for me. Maybe she watches other universes where she sees other Wandas and Pietros and Visions and various friends in the Avengers get murdered, and she becomes numb to it. You could have her say that she saw all their sacrifices be in vain, and it made her realize that the sacrifice she made - giving up Vision and her kids - was also pointless. And yes, she saw countless ways people suffered at the hands of different villains, but the thing was, at least the people who were doing the hurting were the ones in charge. And she decided that she wanted to be the one making decisions for herself.
She had all the power in the world and tried to play by the rules and lost people over and over again. So now she's going to make the rules.
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Just present that on the screen. Sure, maybe the Darkhold is nudging what she sees, keeping her from seeing the paths where she uses her powers to make a positive difference. But we'd still be seeing her make a choice, with a motivation.