WARNING: The following contains spoilers for WandaVision Episode 8, "Previously On," streaming now on Disney+.
WandaVision fans have been producing Mephisto theories at all turns, with many still thinking he might have a role to play despite Agatha Harkness being revealed as Wanda's enemy inside Westview. Some think the demon's hidden in the reality bubble, and others believe he's somehow behind Agatha; however, the latest episode suggests Mephisto has been in front of viewers all along, and he's been pulling Wanda's emotional strings since Avengers: Endgame. In theory, Mephisto could be the director of S.W.O.R.D, Tyler Hayward.
It's safe to say Hayward is a fiend, as he's tried to assassinate Wanda and is adamant that the Avenger is a terrorist. His latest plan involves activating his own reprogrammed Vision to go after her, so it seems as if he's like another General Ross. However, early on in Episode 8, there are hints that he's playing Wanda, wanting her to unleash her chaos magic for some nefarious purpose.
When Wanda comes to his facility in a flashback, she inquires about Vision's body as she wants to bury it. Hayward lets her into his office, but rather than show compassion, he prods and provokes her, actively wanting some sort of response. He also makes it sound like she and the Avengers are partly to blame for the synthezoid's death. More so, rather than hide the dissection, he eggs her on to see the body during its robotic autopsy, as if he's wanting her to lose it.
She does to a certain extent, breaking the glass down before descending to see her beloved being taken apart. It's clear from Hayward's physicality, as well as how he talks down his agents, that this was a success for him. He is pleased -- not scared or ashamed -- to cause Wanda more distress. That says a lot, as he's the catalyst for Wanda to go off to create her reality in Westview, which Hayward proceeds to terrorize himself under the auspices of national security.
Now, these prompts are exactly the kind of behavior one would expect from Mephisto. As Marvel's devil, he knows all the buttons to push to get Wanda to break. Furthermore, he had a part in creating Billy and Tommy in the comics, knowing that losing said family would shatter Wanda. This seems to be an adapted spin on that, and it might be that Mephisto wants or needs Wanda to totally break down, and being Hayward is the perfect way to provoke this.
Agatha has already said Wanda's the Scarlet Witch, a chosen one of all-powerful magic, so Wanda may be linked to Mephisto inherently. After all, her red energy and the new title could tie into Mephisto's look and realm, with other theorists believing he wants to drain Wanda of her magic for himself. It's why he first needs her to max out at full capability. Theoretically, disguised as Hayward, Mephisto could be puppeteering everything and everyone, from Agatha, to Monica, to Wanda, just to ensure all hell breaks loose for his benefit.
Written by Jac Schaeffer and directed by Matt Shakman, WandaVision stars Elizabeth Olsen as Wanda Maximoff/Scarlet Witch, Paul Bettany as Vision, Randall Park as Agent Jimmy Woo, Kat Dennings as Darcy Lewis, Teyonah Parris as Monica Rambeau and Kathryn Hahn as Agnes. New episodes air Fridays on Disney+.