Warning: This article contains spoilers for Stranger Things season 4, vol 1.
Stranger Things 4 pulled off one of its biggest and best twists, and as always, the Duffer Brothers signposted some clues before the big reveal. Across the first 7 episodes, three different narratives were introduced that eventually all tied together in a masterful exhibition of storytelling. The Creel's experience with a 'demon', Vecna's Curse, and Eleven's unearthed memories from the Hawkins Lab combined in a genius way. It revealed that the main villain of season 4 turned out to have been forged right in the Hawkins Lab, beginning his existence as Henry Creel, Victor's son, and going on to become Doctor Brenner's Number One.
A big Stranger Things question finally answered was El's escape from Hawkins Lab, which has been a mystery ever since the pilot episode in 2016. Not only that, but the Lab itself has largely been a mystery, as its covertly unethical practices haven't yet been explored in all their brutal reality, and it was unclear what began it all. Brenner is an ethically grey man at best, and pure evil at worst. Taking children from their mothers to experiment on them in tortuous ways means that Stranger Things must have merely scratched the surface of what went on in the Lab. However, audiences now at least know where it all started; Number One.
Number One began life as Henry Creel, the son of Virginia and Victor Creel, and was imbued with strong telepathic powers from a young age. After murdering his family, he slipped into a coma where he was finally taken by Brenner, who had learned of his suspected gifts from his mother. Brenner, unable to control One's unruly powers, tried the next best thing; replicating them, which, of course, led to Eleven. One couldn't be allowed to walk free, though, thus Brenner implemented him with a power-dampening chip, keeping him in the Lab as an orderly, scared of what might happen if his evil was ever unleashed.
Jamie Campbell Bower Casting
The most obvious clue of Vecna's true identity, perhaps, is Jamie Campbell Bower's casting, introduced officially as the 'Friendly Orderly' who befriends Eleven at the laboratory. He becomes very prominent in some of her earlier memories unearthed by the Nina Project. His featuring such extensive screen time, and being an established actor, hints that his role will definitely have more than meets the eye. Further, Vecna having fairly human features, unlike any other Upside Down monster, hinted that his origins must be more true to life than anything else in the Upside Down. Thus, the introduction of Jamie's 'Friendly Orderly,' a new addition to the Stranger Things cast, drew the two characters together immensely. This, as well as the fact that he is credited in every single episode - even when the 'Friendly Orderly' seen at the laboratory has not appeared in it - should have been a huge hint as to his true identity as the villain Vecna.
Lab Victims
The opening of Stranger Things season 4 proved shocking to most audiences. It shows Eleven, soaked in blood, standing in the Rainbow Room of Hawkins laboratory surrounded by the dead bodies of her brothers and sisters. This sets up one of the biggest bait and switches in all of Stranger Things, leading audiences to believe that Eleven committed such atrocities, which is why she buried the memory, and why she frequently refers to herself as a monster.
It is revealed in episode 7 that it was actually the 'Friendly Orderly' who had gotten to know Eleven that massacred the entirety of the lab, as he is the powerful Number One; not just a myth, but a real person. The victims each share the same injuries - bones snapped in horrifying directions, and haunting bleeding eye sockets. Even El, though stained with blood, has blood leaking from her eyes, correlating with the villain Vecna's curse. Later in the episode, Chrissy's death happens in the same manner. Her arms and legs begin to snap in outward directions, her jaw clicks sideways, and her eye sockets begin to bleed, exactly like the lab victims. All of Vecna's victims share the same signature traits, and with Eleven also sharing these traits, it hints not only at her innocence, but that the evil behind the act came from inside Hawkins lab.
Creel Family Experiences
The heavy focus on the Creel family definitely signposted that something was going on with the family that connected them to Vecna. The plot was largely hinting that Victor might be responsible for Chrissy's murder as her body resembled that of his family, but this was merely a red herring. As Nancy and Robin get the details of the true story, Victor (Robert Englund) recalls that the torment began with dead animals appearing around their land. Alice is the one who finds the first of these dead animals; a rabbit. In the scene, Henry isn't shown, and Alice looks visibly scared, as she immediately runs back to the house after spotting it. Victor recalls that they were ''mutilated'' and discredits the idea that it could be the work of wildcats. This specific description implies that it would have been done by a human. The mutilation of something is a very deliberate act, as well as the fact that the dead rabbit conveniently appears right at the foot of the children's slide, as though it was placed there purposefully. Victor goes on to say it was the work of something ''neither animal nor human,'' which fits Henry Creel/Vecna perfectly in many ways.
In fact, as Victor recounts a lot of his family's demonic experiences, Henry is notably absent. Henry's absence in the story is half explained by the description of him being a quiet, sensitive child, but some subtleties give his evil away. At the dinner table, when Virginia is killed, the camera shows Alice's reaction in its fullest form as she screams and cries. The camera, however, seems to avoid Henry at all costs, because his reaction wouldn't be one of fear, but of pleasure. When Victor tries to break down the door to get the children out, Alice is crying and eager to flee, but Henry just stands there, eerily calm, with no intentions of leaving until he's finished his massacre.
Vecna's House
Before Stranger Things 4's biggest twist happened, Victor told viewers himself that ''all evil must have a home.'' Shortly after the exposition laid by Victor - in which his family home is focused on immensely - Max has her encounter with Vecna. Unusually, the red smokey area she's led is not like anything seen in the Upside Down before. She wanders through the smoke, and comes upon a dismantled house. A grand staircase stands in the middle, the ominous clock that has been chiming floats in the air, and the Creel family door that audiences were shown mere scenes before is also evident. Vecna is also confused at Max's intrusion into the scene, which furthers the point that this is a location that is personal to him. This, perhaps one of the most obvious of all the hints, was hidden perfectly amidst the anxiety-fulled drama of Max's life-saving song scene.
Vecna Spares Henry
After Virginia's brutal death, Victor's daughter Alice was taken next. When Victor returned from his musical trance, he returned to see Alice's body with snapped and bent bones, akin to Vecna's other victims, but Henry was merely unconscious. He seemed, by all accounts, unharmed. He slipped into a coma, even, and didn't appear to have battled a ferocious and murderous demon. Upon this revelation, there really was no conceivable reason for the demon that Victor believed lived in their house to 'spare' Henry. After killing the mother and daughter of the Creel family so violently, why would Vecna leave Henry unharmed? The only plausible reason is that Henry had something to do with the murders, which, of course, turned out to be the case in Stranger Things' lengthy season 4.