One of the theories could be that Will has some supernatural powers that caused the monster to be lured to him even though he wasn't injured. That could also be the reason he could communicate with his mother using the lights.
The Demogorgon Tried (& Failed) To Use Barb
Barb's death implied that whatever the slug does didn't take, so the Mind Flayer reached out to Will instead. It could be that the Mind Flayer needed a younger host, or Barb may have simply died while the Demogorgon was attempting to infect her.
The door begins to unexplainably open, causing Will to run out the back door of the house and into the shed, grabbing a rifle. After a brief moment of tension, something appears behind the boy and abducts him to the Upside Down.
As his strength grew weaker, the Demogorgon abducted Will while unconscious. His body was taken over by vines which could have been keeping him alive for the Demogorgon to feed on later, but he was soon rescued by Joyce and Hopper.
It uses Will because of the connection Will has to Eleven, who would be able to open further gateways to parallel universes so the Mind Flayer would be able to continue this cycle nearly indefinitely. As the Redditor continues, it becomes clear why Will could be the Demogorgon.
Vecna's first big credited kill was Chrissy Cunningham (Grace Van Dien). He followed that murder with canonically terrible driver Fred Benson (Logan Riley Bruner) and basketball star Patrick McKinney (Myles Truitt). But wait, there's more.
Vecna was the one who originally kidnapped Will in season 1.
In episode 7, Vecna is revealed to be Henry Creel (played as a youngster by Raphael Luce), the son of Victor Creel (Robert Englund) and his wife Virginia (Tyner Rushing) shown in the 1950s flashbacks.
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.
Brown is likely right in saying that no one created the Upside Down and that the parallel dimension has always existed. And if the boys' science teacher was right in saying there are “infinite variations” of our world in alternate dimensions, there could even be several versions of the Upside Down out there.
The monster wants revenge after Eleven stopped its spread into our world. It was clearly displayed in season 2 that the Upside Down and its inhabitants harbor a grudge against Will for escaping in season 1.
Using past traumas to break down his victims, Vecna brought Nancy to the pool where Barb was killed by the Demogorgon and revealed her mangled corpse. Vecna then taunted Nancy with her own feelings of guilt - that she was responsible for Barb's death and had forgotten about her.
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.
After defeating the Demogorgon (which may have been acting on Henry's orders), Eleven vanished inside the Upside Down and then escaped — eventually going to live with Hopper in a secret cabin in the woods.
Because Will is the center of its plan, killing him would cause a simplified version of the grandfather paradox: killing Will would erase the opportunity for Will to ever get access to the Mind Flayer and become the Demogorgon.
Unlike in Stranger Things s4 where Eleven loses her powers, her powers are still active at this time, even though she was considerably weakened after defeating the Demogorgon, which is likely how the hunter survived her then-unfocused telekinesis.
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).
Turns out, he needs exactly four dead kids, like the four chimes of his signature clock, as they are his necessary sacrifices in order to open enough small gates in Hawkins, so that he can create that earth-shattering gate in the final episode.
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.
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.
He believed that the construct of time and monotony needed to be destroyed. So, in his mind, by destroying humanity, he was doing it favor. That's what makes him so evil.
In a fit of rage, Henry used his restored powers to brutally murder the other test subjects and workers, and attempted to do the same to Eleven. However, Eleven stood her ground and ultimately overpowered him, sending him through a gate to the Upside Down; there, he gradually transformed into the being known as Vecna.
Mike, on the other side tries to give strength to Eleven, who is at risk as she is captured by Vecna. Eleven gathers her strength and overpowers Vecna, who's just about to kill Max.
At the height of his empire Vecna was betrayed by Kas. Kas managed to destroy the Archlich before his own death, with the Sword of Kas, a lethal blade of the Undying King's own make, leaving only Vecna's left hand and left eye behind. Vecna eventually returned and by 581, had risen to godhood.
Though Vecna is now more powerful than Eleven, at the time she managed to escape through a psychic joust. Kali's escape was far more subtle, happening before the massacre could even take place. The series implies that Kali was able to escape by using her abilities.