Combined with the fact that Dr. 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.
Vecna's Mindscape is separate from both the Right Side Up and the Upside Down - thus, the bodies in the Mindscape aren't real. The aftermath of Chrissy, Fred, and Patrick's respective deaths shows their bodies remaining in the physical world to solidify that they hold a separate psychic existence in Vecna's Mindscape.
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).
He shared his distaste for the controlled structure of society. Vecna considers this to be a prime flaw in humanity. Although his hatred for humanity might have been his prime motive for the 1986 murders there is no denying his victims are fueling him.
Vecna taps into the trauma in a human brain to cause pain while the Mind Flayer doesn't have a human brain, so Vecna's powers become useless against it. However, if Vecna can tap into The Upside Down's hive mind, he may be able to take down the Mind Flayer.
The first reason that Vecna was stronger than Eleven is simple. He was older and more experienced. He was the original subject in Project MKUltra, and using his DNA, Dr. Brenner was able to make or alter the other children so that they would also have powers.
Who Created the Upside Down? In the "Stranger Things" timeline, the earliest we see the Upside Down is during Eleven's time in the Hawkins National Laboratory when an experiment guided by Dr. Brenner results in her accidentally opening the gateway between our world and the hellish dimension on the other side.
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.
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.
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.
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.
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.
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 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.
These visions build over time, and within a space of 24 hours Vecna's victims enter one final trance. There, Vecna taunts them, rejoicing in their heartache and sorrow, before killing them. Vecna's victims die in an agonizing way, lifted high into the air.
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.
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.
"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."
We still don't know why Vecna waited seven years after being banished to the Upside Down before starting his murderous rampage. At this point, I'm not sure we ever will. However, it's not like Vecna was twiddling his tentacle-y thumbs for seven years.
The orderly, who befriended Eleven because she was the most powerful subject, tricks her into removing a chip in his skin. It turns out that the orderly is the mysterious Number One, a man named Henry Creel who came under Brenner's care after killing his parents.
As well as befriending Eleven in order to gain her trust so she would remove his Soteria implant, we learn in the final episode that 001 initially spared El because he wanted her to join him. He viewed her as an ally rather than an enemy because they were "alike".
One manipulated Eleven into plotting her own escape, which also involved getting her to remove the device in his neck. (Henry, however, did not tell Eleven the truth when he said Brenner wanted her dead. Brenner often lied, but his actions towards Eleven were about control and unlocking her powers, not killing her.
Vecna, who absorbs beings' consciousnesses and resultantly feeds off their power, knew he had stumbled upon exactly what he needed to exact his revenge on the world: I saw so many things, then one day I found the most extraordinary thing of all. Something that would change everything.
Sharing more about Eleven's real father Andrew Rich could be a great way to wrap up the story.
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.