Wanda's arc on "WandaVision" is all about learning to handle her grief without destroying everyone. In the end of that show, she comes to terms with her grief, lets go of Vision and their children, and admits that she knew she'd done a lot of bad things and needed to do better in the future.
When we pick up with her in "Multiverse of Madness," though, it seems like she's completely forgotten all of that. Her single-minded determination is to "rescue" the sons she lost, and she's willing to destroy realities and kill anyone who gets in her way. The corruption of the Darkhold could be an explanation, but it's not a particularly satisfying one in terms of character development.
