I can agree that maybe it needed slightly more explanation, but it read to me as a clear case of "Don't pick up the One Ring, its bad". The movie makes clear that reading and using the Darkhold is A Very Bad Thing, and even establishes that Strange doing so once, briefly, is still a thing with consequences. What the movie probably needed was one scene making clear that Wanda found the Darkhold ( maybe in Agatha's library? ), and had unknowing and unrestricted access to it for months, which is what screwed her. She was basically in the same place as Illuminati-World Dr Strange, in terms of "By the time anyone realized the problem it was too late".
Which, note, I would say they *didn't* portray Wanda as irreversibly corrupted. She was in a very bad place, but all throughout the movie there were moments where she hesitated and clearly had *some* suppressed doubts. Thus why the ending could work- she was corrupted and warped by her love of her lost family, and when that motivation itself contradicted her corruption, it broke her free. Which was also foreshadowed by how Illuminati-World Dr Strange, who similarly broke free of his corruption when faced with the destruction of a whole world, thus his helping defeat Thanos in a different way and then submitting to his own death.
