Dr. Brenner (Matthew Modine) clarifies that Vecna "consumes" his victims, as he isn't interested in simply killing them - meaning Vecna takes everything about and from a person.
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 Stranger Things' "The Massacre at Hawkins Lab," Vecna/001 clarifies that the massacre victims are "not gone" and are "still with [him]." Considering Brenner's clarification that Vecna consumes his victims, Vecna stores his victims' essences in his Mindscape for further use.
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's use of his victims' guilty conscious to terrorize his prey is an obvious tool in his arsenal. What is more subtle and powerful is that he uses shame to isolate and control. All of his victims, starting with Henry's father, believe that if people know the truth about them, they will face rejection.
Vecna as a Means for Externalizing Trauma and Depression
Chaiken suggests that while Vecna is a source of horror in Stranger Things, the monster can also serve as a useful vessel for externalizing issues like trauma and depression.
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.
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.
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.
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).
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.
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.
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. Henry then found himself in the care of Brenner, who decided he wanted more kids like the boy.
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.
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.
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.
Mike, on the other side tries to give strength to Eleven, who is at risk as she is captured by Vecna. Eleven gathers her strength and overpowers Vecna, who's just about to kill Max.
This is a reference to the D&D character Vecna, for whom this Vecna is named; the D&D Vecna has a monstrous hand infused with dark magic. The design of Vecna's hand is also a nod to Freddy Krueger, the Nightmare on Elm Street villain who had a huge influence on this season in general.
As the ending of Stranger Things season 4, part 2 made very clear, Vecna, a.k.a. Number One, is far from finished with Hawkins. Yes, he may have been beaten and battered by Eleven, Nancy, Robin and Steve, but he did survive.
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.
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.
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.
After learning how Victor Creel was able to evade death back in 1959, Robin and Nancy deduce that playing the victim's favorite song can help them escape the trance that Vecna places them in, with music and happy memories being the only way to keep Vecna's fatal curse at bay.
Whenever victims become entranced or possessed by Vecna, their eyes form a milky glaze, until they meet their demise as Vecna plucks their eyeballs out. Also, when Vecna's victims hallucinate — an early indicator they've been targeted by Vecna — the people in their hallucinations have milky eyes.
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.