Thomas Shey
Legend
I do think the Darkhold is narratively important to get her there "so quickly". Without it, I could see Wanda getting there eventually, but I would need to see that progression (else we have another Season 8 GOT Dennerys heel turn). But the book acts as a catalyst, accelerating Wanda through that progression and making her more unable to listen to reasonable arguments than she might have otherwise. Considering that Strange used it for like 10 minutes and may already have corruption in his body....it seems very reasonable that a person using it for several weeks (months?) could be completely corrupted.
In fact, we pretty much know at least two other incarnations of Strange fell down completely that way.
Honestly, you can of course have an issue with the Darkhold as plot device, but that's pretty much how its worked out for anyone who messed with it for an extended period. Its well established in prior fiction. Of course they didn't have to give it to her, but once that was set up there's pretty much have to been active intervention on someone's part (say, Strange getting his oar in much earlier) for some awful outcome not to have been likely; Wanda's just been shown as someone who struggles too much with her better and worse angels not for that to be expected, given how the Darkhold puts its thumb on the scale (and frankly, one gets the feeling the nature of her maturing power didn't help).