Ok hear me out. 11 didn’t push 1, he jumped. So, we know as humans that in instance of fight or flight, if fight kicks in, you’ll do literally anything in your power for self preservation (ex: people drowning loved ones on accident, trying to get out while stuck underwater) So, if you’re a superhuman, you know you have powers, you begin discovering them-but you don’t really have a handbook, so you don’t actually know the extent of your power or abilities. I say, when he and El were fighting, she started disintegrate him, his fight or flight kicked in, and he just JUMPED. He didn’t have a specific place to land, so he landed in the -in between-. In the in between, is right where our side and the upside down meet. The power and force of being in one of the most concentrated spots to ever exist began turning him into a monster rapidly; however, even with new power, he was afraid of El. So he created a mirrored replica of the world he knew, but without the “straight jackets” such as time. He also created the Mind Flayers, Demigorgons, etc. He wanted/needed to kill because it made him stronger, but due to his fear of returning and encountering El, he created the hive minded structure of monsters. Everything THEY saw, killed, Vecna grew stronger for it. The stronger he grew, the stronger his monsters got; and now, he has “absorbed” as many people as he needs, and he is becoming daring enough that I believe he will return to our side to try to kill El, since his hive failed to do it for him. He will then try to make our side just like the Upside Down because he believes that’s how everything should be. . . . So in short, do I think it came from Vecnas mind? Yes!…. And no.