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).
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.
Patrick was targeted due to his father's abuse of him. Finally, Vecna targets Max for her trauma from witnessing Billy's death and uses Barb's (Shannon Purser) death as a way to use Nancy.
"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."
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.
Then Eleven—who in this future moment discovers that she in fact does not have the blood of her siblings on her hands—defeats Henry Creel, sending him to the Upside Down where he gets blasted by lightening and burnt to a crisp and, thus, becomes Vecna.
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.
We know Eleven was born in 1971, which means her mom got pregnant with her during the tests, sometime in 1970. A 1959 flashback in the mid-season 4 finale reveals a 12-year-old One (Jamie Campbell Bower). That puts One, a.k.a. Vecna, as 24 when Eleven was born.
She Doesn't Have a Great Aim
After the Hawkins team develops a plan to defeat Vecna, Nancy is in charge to blow his head up with a shotgun. However, either she didn't have a great aim or she was trying to play it safe because she didn't hit him in the face even once, and that may be the reason why Vecna is still alive.
She breaks free, momentarily incapacitates Vecna, and runs with the determination to live toward her friends, dodging debris that Vecna throws in her path. Max hurls herself through the portal, awakes from the trance, and is held by Lucas (Caleb McLaughlin).
Through the window, Max saw her real body levitating above the cemetery, her friends panicking on the ground below. Bracing herself, Max pulled at a vine in Vecna's neck, causing him to writhe in pain and accidentally release her from the vines' grasp.
Once he takes control of the mind, he traumatizes them by showing the visions of their dark past. He takes them to unknown places and gives them a glimpse of himself. The pain of his victims doesn't end here. Every single one of his victims experiences nose bleeding and severe headaches as well.
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.
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 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.
"I walked up to her [as Vecna] and she burst into tears. She wouldn't look at me and she was visibly just disgusted by the whole thing... After she cried and I made it obvious that she knew it was me, one of the things she said was that, 'I knew it was you when I could smell cigarettes,' because I'm a smoker."
What does Vecna want with El? In his twisted mind, Vecna believes that he his helping the world by wiping out humans, as a "predator, but for good." When Vecna asks for El to join him, it's because he also sees her as a predator, "better" than humans.
He's the infamous Number One, but had his powers blocked and worked as an aide in the Hawkins National Lab. He befriended Eleven at the lab and not knowing Vecna was evil, Eleven restored his powers resulting in the massacre and his expulsion to the Upside Down.
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.
This also perfectly explains why Vecna's Upside Down spot is the Creel House. It is the place where he discovered his “purpose” and he can hang out there with his deadly spiders and gain strength from taking lives.
Remember also that we have evidence that powers can be inherited -- El inherited her powers from Terri Ives, for example. So it stands to reason that Henry inherited his powers from one of his parents.
This is because his trusted accomplice, Kas the Bloody-Handed, betrayed him and used the sword Vecna created himself to chop off his left hand and remove his left eye. This ultimately destroyed Vecna. Though they're clouded with cataracts, Vecna in Stranger Things has both of his eyes.
At his empire's height, Vecna was betrayed and destroyed by his most trusted lieutenant, a vampire called Kas the Bloody-Handed, using a magical sword that Vecna himself had crafted for him, now known as the Sword of Kas.
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.