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.
As shown when he was fishing for victims, Vecna can access the memories of adults as well. In fact, his first victim as a child was his adult mother - clearly, he's more than capable of targeting them. However, Vecna could possibly be targeting teenagers over adults because they're easier for him to manipulate.
"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."
Harnessing psychic abilities, Vecna can detect humans in Hawkins, and just like Eleven, he can use telekinesis to kill his opponents. Unlike Eleven however, he can also use advanced telepathy and create false visions in his victims, which makes him even more dangerous.
Once the monster catches them, as he inevitably does, Vecna kills them from the inside out by taking their soul while their body in the real world is broken in horrific angles and their eyes are gouged out.
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.
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.)
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).
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.
(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.
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.
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.
The theory is that Karen Wheeler might be Vecna's sister, Alice Creel. As we know, Vecna is actually a man of many names, including 001, Peter Ballard and Henry Creel. We saw Henry and Alice together as children briefly in a flashback and it is believed that Henry killed his sister along with his mother.
As Sink put it: “You kind of learn in the season that he targets people that are in a real [bad] place, and Max is obviously one of those people. Just with everything with Billy, so she was kind of like the perfect target.
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.
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.
Vecna's Hand
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.
In the D&D realm, a defining feature of Vecna is that he's missing his left hand and left eye. 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.
Together, their powers combined are the exact powers of 001/Henry. Fans are now theorising that the powers of both 011 and 008, who have the combined power of 001, is what is needed to finally match and defeat Vecna. To put it simply, 011 + 008 + [with the combined power of] 001 = 020, a.k.a. Erica's critical hit.
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 offers her his path as a sort of freedom, as they reshaped the world in their image.
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.
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.
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.
Via his 'curse' power, Vecna could make other conscious beings hallucinate and bring them into the mindscape. On March 24, 1986, Max Mayfield entered the mindscape while afflicted with Vecna's curse, but managed to break out of the hallucination and escape to reality.
Though Fred appeared to be bright and level-headed, he harbored extreme guilt for his role in the fatal 1985 car accident, and believed many people look at him as a "murderer." This guilt and mental unrest made him a target of Vecna.