During Henry's time as an orderly at Hawkins Lab, he took a particular liking to one of Brenner's test subjects, Eleven. After witnessing her being bullied by the other test subjects, he believed both of them were similarly misunderstood, and also believed Eleven to be the most powerful of the test subjects.
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.
That's because Vecna wanted to take and understand Eleven's powers. In particular, Vecna wanted to know how Eleven was able to open gates to the Upside Down. So, rather than killing Eleven, the Spider Monster lodged itself inside Eleven's leg to sap her powers and learn her secrets.
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 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.
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.
One/Vecna had his powers blocked and was working as an aide at Hawkins National Lab where he befriended Eleven. Not knowing that One was evil, Eleven restored his powers which he used to gruesomely massacre the entire lab.
And he saw the possibility for escape when he realized the extent of Eleven's untapped potential. 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.
"I walked up to her [as Vecna] and she burst into tears. She wouldn't look at me and she was visibly just disgusted by the whole thing... After she cried and I made it obvious that she knew it was me, one of the things she said was that, 'I knew it was you when I could smell cigarettes,' because I'm a smoker."
What Does Vecna Want? Vecna's goal is to open as many gates to the Upside Down as he can, in order to help the Mind Flayer. The Mind Flayer, is, of course, the ULTIMATE big bad, and he's looking to take over the world.
In the series, Eleven is shown moments from her past at Hawkins Laboratory, right up until she manages to banish Vecna into the Upside Down.
Vecna successfully stole Eleven's powers when controlling the Mind Flayer in Stranger Things' season 3 ending, which finally allowed him to emerge stronger than Eleven and enact his plan to take over Hawkins with the Upside Down.
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.
We find out in episode 7 that just like how everything in the Upside Down is connected, Vecna is also deeply connected to Hawkins and Eleven. Nancy (who is still in the Upside Down with Steve) ends up under Vecna's control, and she starts seeing visions about Barb's death.
His first three victims — Chrissy Cunningham, Patrick McKinney, and Fred Benson — are all peers of The Party, people they know from school, which makes Vecna's terror all the more real.
Then Eleven—who in this future moment discovers that she in fact does not have the blood of her siblings on her hands—defeats Henry Creel, sending him to the Upside Down where he gets blasted by lightening and burnt to a crisp and, thus, becomes Vecna.
Played by Jamie Campbell Bower, each prosthetic was glued to his skin with medical adhesive by prosthetics designer Barrie Gower and his team. For the finishing touch, Vecna was covered in "glossy slime to make him really, really glossy and wet looking," according to Gower.
Joe Manganiello's character Arkhan fought against Vecna with the adventuring party Vox Machina in the web series Critical Role. He then stole the Hand of Vecna and replaced his own hand with the artifact.
However, within the red smoke of Vecna's Mind Lair, it wasn't the song itself that saved her, but the memories it conjured up. What broke through to Max was the happy, loving memories of her friends, particularly Lucas and Eleven, and these were what truly allowed her to break free of Vecna's grasp and escape.
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.
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.
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.
Sharing more about Eleven's real father Andrew Rich could be a great way to wrap up the story.