The development of a TV series such as WandaVision isn't always smooth sailing. Ideas for a show eventually shift and change into the final product we eventually see. In the case of WandaVision, multiple story aspects changed over time, including the character of Agatha Harkness.
In the final product, Agatha Harkness is the shadow in the background, an evil puppet master pulling strings. However, writer Jac Schaeffer explains that her original role was to teach Wanda how to use magic.
Agatha Harkness wasn’t always a dog killer
Agatha was pretty damn evil in the final version of WandaVision. She killed Sparky, kidnapped children and tried to steal Wanda's powers. Granted, Wanda did mentally enslave a whole town and make them feel her grief, but Agatha Harkness was the big villain! With that said, the third act villain was supposed to be a mentor character.
"In the original conception, Agatha’s character was more in the mentor and magic-expert space. One of the things that never changed was that in the finale, Wanda would have to say goodbye to Vision. In my original notion of it, that goodbye was like a final binding spell that she had to do. And it was tied to a spell that Agatha had taught her early in the series, where a gravy tureen had shattered, and Agatha taught her this very basic binding spell. In the end, what she has to do is integrate her trauma, and she has to bind Vision back to herself with that spell. Agatha ended up becoming more of an antagonistic force, because we needed that in the series."
Well, that explains a lot
As mentioned previously, Agatha Harkness wasn’t really much of an antagonist for most of WandaVision. While the show claims it’s been “Agatha All Along", it really was just Wanda. Harkness' original role as a teacher of magic does make the weak villain moments make a lot more sense.
Nevertheless, Agatha was at least an entertaining on-screen presence. Hopefully, we'll be able to experience the full reign of her villainy in a future MCU project.