When Will was trapped in the Upside Down and captured by a Demogorgon, the Mind Flayer became interested in Will and decided to use him for his own purposes, showing no care for him.
Will's powers on Stranger Things started in Season 1, after he was taken into the Upside Down. The exposure to that underworld led to him having True Sight — aka visions of the Mind Flayer.
Human Possession
For the Mind Flayer's hive mind to work, the monster must infect an individual. In doing so, the victim would act as a host body for the Mind Flayer's possession. This occurred with both Will and Billy Hargrove, causing the characters to serve as spies in the primary dimension.
It uses Will because of the connection Will has to Eleven, who would be able to open further gateways to parallel universes so the Mind Flayer would be able to continue this cycle nearly indefinitely. As the Redditor continues, it becomes clear why Will could be the Demogorgon.
They are connected because Will was the first victim. That's it. Eleven is Vecna's true foil, an anti-Vecna, if you will. We learned a lot about Eleven's connection to Vecna and how their brutal fight in Hawkins Lab manifested the Upside Down.
While intending to get his revenge on Eleven and Brenner, Vecna also goes after victims who have dealt with traumatizing experiences in the past (reminiscent of his childhood and his time at the Hawkins lab).
Re-watching the episode where Will Byers goes missing, there is a lot of evidence suggesting that it was Vecna who abducted Will and trapped him in the upside down. Many Stranger Things fans who re-watched the scene pointed out that they could hear the chimes of the grandfather clock.
The Demogorgon Tried (& Failed) To Use Barb
Barb's death implied that whatever the slug does didn't take, so the Mind Flayer reached out to Will instead. It could be that the Mind Flayer needed a younger host, or Barb may have simply died while the Demogorgon was attempting to infect her.
Vecna's first big credited kill was Chrissy Cunningham (Grace Van Dien). He followed that murder with canonically terrible driver Fred Benson (Logan Riley Bruner) and basketball star Patrick McKinney (Myles Truitt). But wait, there's more.
There's been debate as to whether or not this scene implies that Eleven created the Upside Down or merely opened the first gate to it. But according to Millie Bobby Brown, the Upside Down existed before El ever meddled with it. “I don't think that she created the Upside Down,” Brown told Variety in June.
What does the Mind Flayer want? According to what Flayed Billy (Dacre Montgomery) reveals during his final showdown at the end of season 3, the Mind Flayer's current plan is to kill Eleven because she is its one true enemy and the only force who can destroy it.
In episode 7, Vecna is revealed to be Henry Creel (played as a youngster by Raphael Luce), the son of Victor Creel (Robert Englund) and his wife Virginia (Tyner Rushing) shown in the 1950s flashbacks.
The Mind Flayer has superior intellect to that of a human. He knew Billy was the perfect host out of everyone else he flayed, and was likely going to save Billy for last.
Vecna taps into the trauma in a human brain to cause pain while the Mind Flayer doesn't have a human brain, so Vecna's powers become useless against it. However, if Vecna can tap into The Upside Down's hive mind, he may be able to take down the Mind Flayer.
Whenever Vecna kills someone, he "consumes" them, according to Dr. Brenner; that means he acquires their consciousness and power. By using his own powers, Henry/One took control of the Mind Flayer and obtained all of his powers, thus becoming stronger and in control of the Upside Down.
Vecna prays on those dealing with trauma, which is why Max, who recently lost her brother, Billy, was the perfect victim. While she initially got away from him with help from her friends and her favorite song, she wasn't as lucky at the end of the season when Vecna captured her again.
Brenner. The episode culminates in Eleven unknowingly freeing One from Brenner's control, allowing him to unleash his true power on the staff and children in the lab, killing everyone except for Eleven and Dr. Brenner, who is only knocked unconscious.
As Vecna, he uses these mental abilities as a way to psychologically weaken and lure in his victims – much in a way that Pennywise, Freddie Kruger, and Pinhead do. (All of which have been cited by show creators the Duffer brothers as inspirations for the villain.) But he does have a surprising weakness: music.
A few days later, Eleven, while immersed in a deep psychic state, scoured the Upside Down to find Barb and Will Byers. She discovered Barb's body, witnessing a slug-like creature slithering from Barb's mouth and across her face.
In season 4, Peter Ballard notes that Kali was no longer at the lab in 1979. Given Kali's powers, it seems pretty self-explanatory as to how she was able to escape the lab. She either made herself invisible or was able to escape after manipulating the minds and visions of the guards.
Seeing as Barb served as a symbol of innocence, viewers were upset about her sudden death. A "Justice for Barb" movement came together after the character was killed, and seemingly forgotten by the town by the end of Stranger Things season 1.
Essentially, Vecna hates humanity, and wants to take over their world. He had hoped that Eleven would help him achieve this goal. But when he realised she wouldn't, Vecna instead used her.
After a certain period of time, Vecna will finally kill the victim by placing them into a comatose-like state, in which they will be unable to respond to outside stimuli.
Eleven can take control of the Upside Down from Vecna, but she will still need to find a way to shut down the Mind Flayer and the Upside Down. Stranger Things season 4 already showed audiences how Eleven can defeat Vecna in the Netflix series' final season.