Ultimately, it is revealed in Stranger Things season 4's finale that the reason why Vecna was killing Hawkins teenagers was to create a massive gate that merges the Right Side Up with the Upside Down - and he specifically needed four bodies to do so.
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).
In the real world, he makes the victim levitate while in their trance, before snapping their bones and neck, and crushing their eyes. The murder site then becomes a small gate into the Upside Down, as part of a long term goal to apparently bleed both dimensions into one.
In simple words, Vecna needs a total of four gates to take over Hawkins and rule it. Now, you must be thinking, why is Vecna killing people if he can take over the complete Hawkins? Well, in one of the previous episodes, it was revealed that with each person Vecna kills, he creates an opening to the real world.
Vecna lives in the Upside Down and preys on people's past traumas and guilt. The monster curses its victims, making them relive their trauma in progressively more gruesome ways until it violently kills them. (And we mean, like, really violently.)
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.
What Does Vecna Want? Vecna's goal is to open as many gates to the Upside Down as he can, in order to help the Mind Flayer.
Vecna draws power from sad and angry memories and that is the main reason he targets traumatized teens. Those teens are a well of power just there to be absorbed. This is not just a theory, Vecna himself told Eleven about the power that angry and sad memories hold.
Realizing he had tremendous psychic power, he haunted his family with visions before ultimately killing most of them. His father was framed for the murders and locked away in a mental hospital as a disturbed serial killer.
As a child, Vecna killed two people: his mother and his sister. His next massacre as One was far deadlier. By the end of Stranger Things 4 Part 2, we learned that One, and not Eleven, was responsible for the bloodbath at Hawkins Lab.
Brenner (Matthew Modine) states in the Stranger Things episode "Papa" that Vecna "consumes" everything about his victims, Vecna targets traumatized victims because it builds for him a well of sad and angry memories to draw from. With each victim, he can become gradually more powerful.
Vecna's obsession with time appears to be linked to his hatred of humanity. He views time as a human imposition on the natural world; an attempt to impose order on nature.
On the final roll of the dice in Eddie's D&D campaign, Erica manages to roll a 20. It's a critical hit, the party finally defeat Vecna and win the campaign.
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.
What was Chrissy's Trauma? Chrissy's life appears to be free from flaws on the surface, but in reality, she is struggling terribly to conceal her depression and self-image stemming. She was struggling with these issues, which resulted in her developing an eating disorder due to her mother's verbally abusive comments.
Vecna was the one who originally kidnapped Will in season 1.
The episode then cuts to Vecna in 1980s Upside Down, zooming in on his arm, where a tentacle slides away to uncover 001 tattooed on his wrist. So Vecna is 001, who is also Victor Creel's son.
After taunting her with incessant chiming from a grandfather clock and appearing before her in hallucinations, Vecna eventually kills her in an incredibly gruesome way. First, she went into a trance with her eyes rolling into her head, then she began to levitate before her eyes were removed and bones were snapped.
More like this. After previously having tricked Eleven into destroying the chip, Peter goes on a killing spree in the lab until a confrontation with Eleven ended in her casting him into the Upside Down - where after falling through various dimensions and sustaining various injuries, Peter was transformed into Vecna.
He developed a hatred for humanity, believing humans are "a unique type of pest, multiplying and poisoning our world, all while enforcing a [cruel, oppressive] structure of their own." He also found a connection with black widow spiders, who prey on weaker species and keep them from overpopulating.
The theory predicts that Vecna wants to regain his human form, and he will use Will to do so in Season 5. Knowing that Vecna singled out Will in Season 1, it's a reasonable theory, but it might go too far. After all, Vecna already has a human form, and he has vowed to eliminate humanity.
"Vecna seems to be actually intrigued by Nancy being able to figure him out, to be able to get this close to the truth... I think this is more Vecna's narcissistic traits that pull him to Nancy, why he wants to give her this information."
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.
Eleven has a memory during this episode and realizes that 001/Henry Creel (Jamie Campbell Bower) wants her to join him on his evil journey, and she banished him to the Upside Down. 001 then transformed into Vecna, the horrifying Stranger Things villain.
Henry Creel, also known as One, the Mind Flayer or Vecna, is the main antagonist of the Netflix original series Stranger Things, serving as the unseen overarching antagonist of Season 1, the unseen main antagonist of Seasons 2 and 3 (in his Mind Flayer form) and the main antagonist of Season 4 and will return in the ...