Heads up, we'll be talking SPOILERS for the whole first season of Stranger Things. You should really finish it already.
As far as fan theories go, the whole "Hero is really the VILLAIN!" thing is pretty weak. But when you apply that idea to Stranger Things, it makes a lot more sense than usual. Uproxx compiled some convincing evidence in the video above (complete with lovely illustrations) that implies that Eleven and the monster are one and the same. Now, we do see the two together in the same room, but that doesn't mean that the monster couldn't be something like a psychic manifestation of Eleven's childhood trauma.
Let's start with what we know about the monster. The boys named it after the Demogorgon, one of the biggest bads in all of Dungeons and Dragons.
Right off the bat, we can see Demogorgon has two heads. According to the lore, these are controlled by two entirely different personalities, one called Aameul and the other Hethradiah. Despite the pair combined being known as the "Prince of Demons," these two minds hate each other to the core. If they could, they'd kill each other, but they know that without one the other can't exist. Sound like anyone you know? If Eleven and the monster are indeed inseperable like the Demogorgon's heads, that might explain why they both vanished at the same time in the finale.
It's also worth noting that the canonical first sighting of the monster (that we know of) was not in the upside-down, but in a completely black space.
Instead of the nightmarish Giger-esque mirror world, the monster appears in what seems to be Eleven's mind -- and at a time when she's under tremendous pressure in a harsh environment, no less. If the monster was going to show up at any time, it would be now.
While the Demogorgon is specifically called out several times in the show, an allusion to a specific X-Men character is a little more subtle.
In the first few minutes of the opening episode, the boys try to race back home for their pick of the loser's comics. Will says "I'll take your X-Men #134!" Fans know this as one of the turning points for Jean Grey, the telekinetic powerhouse who's struggling with a malevolent deity inside her. That should probably sound familiar, too.
The parallels get even more explicit. In the issue, Jean Grey slams an enemy against the wall with her powers and pins him there -- just like Eleven does to the monster.
Granted, in Jean Grey's case the villain "Mastermind" isn't exactly a separate part of her created by her unconscious mind. But we can see that Eleven and the monster are connected when they both reach their hands out at each other, just before disappearing.
That isn't explicit enough for you? Okay. What about the moment just after Elle saves Mike from jumping off the cliff? She's on the ground, exhausted, and says this:
Though most people probably took this as a metaphor for Eleven's guilt, we should remember that this is a girl who usually says what she means. After all, friends don't lie.