So between the time Wanda has her full on break (aka Wandavision) and reading the darkhold, I don't know if there was really a chance to intervene. I mean Wanda beats up Sword, beats Agatha, takes the book and leaves. She really doesn't hang around as far as we can tell, she pretty much immediately goes into an isolated area and starts reading the evil book. This is one reason I'm not on board with the idea that "Wanda let everyone go and so is back on the heroic path". I mean the very last thing we see in Wandavision is Wanda has literally gone from torturing people to isolating herself and reading a dark book.... its hard to argue that she is trying to go down any redemptive path there.
I think its a grey area. Obviously, staying away from the dark magic book would be a better idea--but at that point she also knows ignoring the unknowns in her powers is not a great idea either, and to the best of her knowledge, the Darkhold may be the best source of information about how to manage them.
Keep in mind, again, while she knows its a book of dark magic, she does not know about the specifically corrupting elements of it; that's something I think people tend to project on her because we have a privileged frame of reference and know more about it than she does. Even Strange doesn't know about that until Wong tells him about it.
Now an outstanding question is, was anyone helping her before the break? We certainly don't see anyone helping her which is not great, and is certainly a tragedy and a pretty damning criticism of her fellow avengers.
I think the weakest part of the movie, though one that can be rationalized, is that after the Return, everyone seems to have kind of gone their own way.