Henry killed his mother and sister in the Creel house, but overexerted himself, collapsing into a coma when he tried to kill his father. The CIA launched a secret mind control program in 1953 that is the root of the story of Stranger Things. There is a chance that Vecna's Gates are causing some sort of instability that could lead to a collision of the pocket dimensions with the real world. All life in the Upside Down is bound together by the Mind-Flayer's hive mind, and at some point, Creel became part of it.
The kids think Vecna has become the Mind-Flayer's "five-star general," but in reality he has actually taken control of the entire hive mind. He lived in the Upside Down for years, making his home in a twisted version of the house he had grown up in. When he used his powers, it's possible that the parts of Creel's brain that were active when he used his powers were the same ones that were active when he was a child. telepaths tend to have something of a god complex, which is why Henry Creel shares the common character flaws of psychics in those comics. Henry woke up and found he was in the care of a doctor. He is a telepath, able to enter the minds of others and probe their deepest fears and darkest memories; Creel also possesses Kali's ability to project illusions into the minds of others.
Henry Creel is the main villain of the fourth season of Stranger Things. It's not clear how he realized that certain procedures could be used to make sure children were born with the ability to use telepathy, telekinesis, and illusion casting. It's unclear why Dr. Brenner decided this was safe, but it's likely he was manipulated by Creel, a smooth talker who knew how to twist people's better natures against them even without his powers. The first test subject was kept a secret from the other children.
Vecna has the ability to open Gates between the Upside Down and Hawkins because he shares all Eleven's powers. Vecna is the new villain from the Upside Down and Number One is it.
It's possible that the earthquakes shaking the Upside Down are a clue to Vecna's endgame. Vecna establishes an initial contact to begin tormenting them, forcing them to confront their worst nightmares, and then killing them with his telekinesis. Creel was unstable and unreliable, even though he tried to control Henry. When he was a child, Creel was able to change the minds of others. She was able to open a window through her powers and blast him out of this universe. If this is true, there could be an open war between Hawkins and the Upside Down.
The experiments being conducted by Dr. Brenner are supposed to be an extension of Project MKUltra, a project that has threatened the entire world. It is possible that Creel would have a natural bond with those he considered his own kind, and that this could be a way to redeem him. He developed a close friendship with Eleven because of her naivete and the power she had.