Unfortunately I think the answer is just that it's an MCU production and there has to be a big fight at the end, it's part of the formula.
Personally I think they could have gotten away without it, considering that there was almost no fighting for the first eight episodes and it was still hugely popular. Maybe that will give Feige the confidence to someday do a series without the big final battle, but this was the MCU's first (originally the second) streaming series, and they couldn't bring themselves to break the formula quite so much.
The MCU Agatha may be "opportunistically evil". By that I mean she isn't on villain mode 24/7. She's been around since the 17th century. There were heroic beings on Earth before Captain America(Eternals? Ancient One?). The fact that she's stayed of the radar from do-gooders for centuries mean that if and when she does something perceived as "evil" she gets in and out and covers her tracks or uses pawns.
I suddenly have to urge to place her and everyone else in WandaVision in an alignment chart (there's one in reddit already where she is CE). Like by your definition, would she be neutral with evil tendencies?
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Obviously Agatha didn't feel the need to help the heroes vs. Thanos and the Black Order.
On another issue. Do folks think it's possible that the cabin Wanda is in at the end is Agatha's? Perhaps Wanda was finally able to read Agatha's mind before she changed her back to Agnes. She said ,"You live HERE now." Could that be interpreted as Wanda finding out where Agatha lived and was able to stay off the radar for so long.
Could be, but it's been pointed out that we hear a bit of Wanda's Sokovian lullaby on the soundtrack in that last scene, so she might have gone back there (the country was destroyed, so apart from being her home it would be a perfect place to isolate herself).
Also I recall Agatha's car had Connecticut license plates, so she's probably from there or somewhere else in New England.
Maybe Wanda found a Zemo property. That would be karmic. Also, a middle finger to him for sowing the seeds of her 'pseudo-family's "divorce."
Agatha was used because they wanted to show how Wanda Maximoff became the Scarlet Witch. I think in all the previous Avengers movies she wasn´t the Scarlet Witch because of the Fox rights, but Disney buying Fox out resolved that. So finally, it was time for Wanda to become the true Scarlet Witch.
Based on what we've seen so far, I think MCU Agatha is chaotic evil with neutral tendencies. Comics Agatha is chaotic neutral. MCU Agatha seeks knowledge and power for only for herself. I suspect that she views the whole good/bad thing as irrelevant to her so long as she is able to become a stronger witch. So far, all we've actually seen her do on screen are evil acts. She killed her mother and coven members when they caught her seeking forbidden knowledge. She lured Wanda into a trap to study her and steal her power. Her mother said that Agatha is not capable of being good. Since comics-Agatha is not an evil character, I'm hoping that MCU Agatha is fleshed out more. I think she'd be more interesting as a chaotic neutral character.
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I'm not sure if Marvel wasn't allowed to use the name -- in all the closing credits of her movies, she's billed as "Wanda Maximoff/Scarlet Witch."
So they could say it, but they never had a reason to. I think Feige may have wanted to wait until they revealed that her powers were magic and that she was actually a witch.
I wonder why they chose Agatha Harkness instead of Morgan LeFay. Maybe they wanted to save her for the big screen. Though I heard she appeared on 'Runaways.'