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. By being in the Mindscape, Vecna can do what he will with his victims outside their physical bodies, including the opening of the giant Gate.
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.
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 puts a curse on his victims by finding people who are dealing with tragedies and negative feelings like low self-esteem. When he has begun his process, his victims feel sick, their noses bleed, they have headaches, and they see things that aren't really there, such as a grandfather clock.
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.
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 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.
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.
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.
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.
While he lives in the Upside Down, Vecna can psychically reach into the human world, kind of like the Mind Flayer's connection with Will. He makes his presence known in Hawkins by murdering the town's teenagers in a truly gruesome fashion, all through mental connections (which is especially terrifying).
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.
Despite his Pure Evil status, Vecna's actor, Jamie Campbell Bower, stated that he sympathizes with his character, as he believes he snapped due to feeling disenfranchised by society and that he did have some genuine care for Eleven until she rejected him. However, the show itself doesn't confirm this.
(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.
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.
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.
What is the significance of 4 for Vecna? The new season shows that the four chimes of the grandfather clock symbolize the four victims Vecna needs to claim in order to enter our dimension and take over humanity.
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. Obviously, Nancy was trying to play it safe. She even cut off the muzzle of her shotgun so she could have the best odds.
We still don't know why Vecna waited seven years after being banished to the Upside Down before starting his murderous rampage.
One of the most important sequences in this episode featured Vecna showing Nancy the real circumstances of the “eyeball murders” in the 1950s, with flashbacks indicating that young Henry Creel murdered his family when his mother tried to get him psychological help and he intentionally framed his father Victor for the ...
It seems that the reason why Vecna wasn't killing for the seven years after the Hawkins Lab massacre was because he was waiting to become more powerful as the Mind Flayer and Demogorgons took more victims, as he was likely weakened after being thrown into the dimension.
He didn't name himself Vecna, the kids did. They named him after a DnD character who's a wizard, since Vecna's curse sounds like something a dark wizard would do.
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 finale, Will reveals to Mike that he has retained some kind of link to the Mind Flayer, and that he can "feel" that Vecna is still alive.
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.