The ending of Doctor Strange in the Multiverse of Madness closes Wanda's story and opens a new problem for Strange. Wanda gives her life to destroy the Darkhold everywhere, America is safe with Strange and Wong, and Kamar-Taj is rebuilt while America trains there.
Why does Wanda bring Mount Wundagore down on herself? In my opinion, the key is the scene in the other universe where Billy and Tommy are frightened of her. Olsen's Wanda has been through Avengers: Age of Ultron, Captain America: Civil War and Avengers: Infinity War, and this is the first time the saga makes her face herself through the eyes of the children she wanted back. She does not lose a fight, she loses an argument with her own conscience, and destroying every copy of the Darkhold is her answer to it.
The third eye is the most telling detail for Strange. He has grown it as a side effect of using dark magic, and the film does not treat it as a trophy. It is a physical bill for borrowing a dead body from a ruined universe. It could be argued that this is the film's real answer to Wanda: she used forbidden magic and lost everything, while Strange used it and kept going, with a visible mark.
There is a useful comparison with Doctor Strange (2016), where the villain Kaecilius steals pages from a forbidden book and Strange is still learning from the Ancient One and Mordo. Here the forbidden book is the problem again, but Strange is the experienced sorcerer who has to decide how far to go with it. Grief as a motive also links forward to Thor: Love and Thunder, where Gorr, a father who loses his daughter, takes up a sword that kills gods.
The last beat is the Clea scene. She appears, tells Strange that his actions caused an incursion between universes, and he follows her into the Dark Dimension. That sets up consequences rather than resolving them. I read it as the film saying that surviving the Darkhold is not the same as being forgiven by the multiverse.
Behind the screen, the tone owes something to the change of director. Scott Derrickson left over creative differences and Sam Raimi took over, with Michael Waldron writing the script after Jade Bartlett's earlier draft, according to TheWrap. For where the saga heads next, see avengers doomsday.