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.
Vecna's powers allow him to form psychic connections with people in reality, especially teenagers dealing with trauma and mental health issues. Vecna develops these connections with young and vulnerable teens so that he can get out of the Upside Down and gain access to the real world by opening four gates or portals.
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.
The exact details of this event are unclear, but it is believed that Vecna killed his wife and children in order to gain more power. Some sources suggest that Vecna's wife and children were also powerful wizards who posed a threat to his own power.
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.
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.
In episode 9, we see Eleven piggyback her way into Max's memories and battle Vecna from within his Mind Lair. Vecna eventually gets the upper hand and Eleven loses. Despite somehow saving Max from death, Vecna's masterplan worked as the Upside Down finally tore its way through Hawkins.
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.
Curiously, he also seems obsessed with time, projecting strange visions into his victims' minds—including of the grandfather clock from his home in the Creel House. Vecna's obsession with time appears to be linked to his hatred of humanity.
3) Around 15 children from Hawkins Lab
Due to his involvement in Dr. Brenner's experiments, Henry did not kill anyone else until Eleven removed his Soteria implant in 1979. As soon as he was free, he killed all the remaining lab children (excluding Eleven and Eight/Kali, who previously escaped).
(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.
And while his usual modus operandi is to embody his victims' negative self-talk (another very real psychological phenomenon), this time he is telling Nancy his plans and then releasing her so she can report back to everybody—Eleven included—that the end is nigh, with the goal of discouraging and disempowering them.
Netflix Vecna has plans for world domination. 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.
In the visions of her trauma induced by Vecna, Chrissy's mother appeared both mentally and emotionally abusive towards her daughter - and it's implied her comments about Chrissy's figure caused her daughter to develop her eating disorder.
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.
He is terrifying because he has access to secrets and the guilt that accompanies those secrets. Vecna's use of his victims' guilty conscious to terrorize his prey is an obvious tool in his arsenal.
Vecna was inspired by mould
But according to Gower, they also “looked at a lot of sea life, moulds and animals to create the look, feel and consistency of his skin”. Gower says the Duffer Brothers were also “very adamant that Vecna be glossy and shiny, that he looked as moist and wet as the rest of the Upside Down”.
"The reason why Vecna only has a single eye and hand is due to a betrayal by Kas", who used the sword against his former master. "In the third edition of Dungeons & Dragons, the Sword of Kas is a +6 unholy keen vorpal longsword that grants a +10 modifier to the Strength score of its wielder".
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.
Vecna adopts psychological warfare to target his prey – which, you could argue, very much serves as an allegory for trauma, depression and mental illness, which is all too real.
At the same time, Nancy, Steve and Robin reach Vecna's body in the Upside Down and throw multiple Molotov cocktails at him before Nancy shoots him, sending his blazing body flying out the window of the Upside Down's Creel House.
In the game, Vecna seeks power through artefacts, while in the series, he enhances his powers by consuming the essence of his victims, like the other gifted children in the lab, and absorbing their powers. When he took over the Mind Flayer, he gained control over the Demogorgons and other monsters.
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. "The theory is that Brenner knew that none of the kids, numbers Two through Ten, were as strong as number One," one of the podcast hosts said in the TikTok clip.
They rush to find Vecna's body and burn it down. With this Vecna's soul turns to ash in front of Eleven. Max's dies in Lucas' arms, thus getting Vecna his fourth kill. The entire town opens up to Upside Down, with all portals dissolving.
By the time he was 12, his mother had decided to call Dr. Martin Brenner to help figure out her son's strangeness. This enraged Henry, who later murdered his mother and sister using his supernatural abilities.