I'm a bit confused where you're getting this.
Wanda literally says "See the difference between us is you did this on purpose" and Agatha kinda just shrugs it off. It's pretty heavily implied in my opinion that she knew fully well if she was caught she'd be able to drain the power away for her own use. Maybe she didn't want to do it to her own mother (unless that was just like, the coven's mother, I dunno), but she showed little remorse after even.
Wanda on the other hand, was all unintentional. Was it horrible still, yes. But she didn't do it for the sake of being a villain, she had no idea how she did it in the first place, much less how to undo it.
And past that, Agatha was just assaulting her mental state. It wasn't about healing Wanda, it was about figuring out what Wanda was. That's all Agatha wanted to know, she didn't care to "heal" her. It was constant mental assaults to basically manipulate Wanda into giving up her power for Agatha's own desire. She wanted to mentally break Wanda so she'd be vulnerable.
And as far as it's been shown, it wasn't about taking her time to take Wanda's magic, it was about that she has to drain it while it's being casted at her. So she couldn't do anything but wait for Wanda to do that, keep in mind every time Agatha drained someone's magic it was only after they casted it at her.
I mean, she had no issue using the same kind of magic that Wanda did to control someone. Nowhere was it implied that Agatha wanted the magic for any good reason, just that she saw that Wanda has a hell of a lot more magic power than she does.