It is not clear if Eleven (Millie Bobby Brown) is dead on the last episode of "Stranger Things" Season 1. A video explained a theory that Eleven and the monster are one.

A video that explained theory about Eleven and the monster, is came from Uproxx Youtube channel. In the last episode, Eleven faced the monster for a final fight. As she raised her hand, the monster raised its hand, and Eleven said, "I am the monster." This scene seems like a reflection of each other. When the monster disappeared, El also gone.

In the story, El was a prisoner of Hawkins Lab, where she was dehumanized and experimented on to enhance her telekinetic skills. The theory also suggests that the monster is actually a physical manifestation of all her fears and terror that she experienced in the Hawkins lab, which then entered over into the reality.

The supernatural-drama series actually features the upside down world, which is completely dark and empty space, where El entered when she was being placed in a deep trance state.

Another "what if" of the theory is, instead sending El outward into another dimension, she actually entered inward into her consciousness. So the monster she met could be the part of her.

The last scene of "Strangers Things" Season 1 is when Will (Noah Schnapp) puked a creature that seems like a monster. This scene made viewers wonder about what will happen next. 

"Stranger Things" Season 1 was premiered in July on Netflix. In a recent report, this series was reported  most-watched best-reviewed original series, according to Variety. On its 35 days on the streaming service, it averaged 14.07 million adults 18-49.  Among the series on the streaming service, the supernatural drama series ranks ahead of "Making a Murderer," which averaged 13.35 million, "Daredevil" Season 2  with the average of 13.35 million), "Jessica Jones" (6.26 million), "Unbreakable Kimmy Schmidt" Season 2 (6.08 million), "The Ranch" (6.01 million), and "House of Cards" season four (5.67 million),