In Stranger Things, the show never gives a single, definitive in‑universe reason why the Demogorgon kills Barb quickly but keeps Will alive, so the answer comes from what is shown on screen plus how the story is structured. Most explanations mix basic plot logic (what the monster is doing) with narrative logic (what the writers needed the characters for).

What happens to Barb

• Barb is grabbed at Steve’s pool, pulled into the Upside Down, and is found later in a nest area already dead and decaying, with a slug-like creature emerging from her mouth.

• This suggests the Demogorgon used her as food and/or as an incubator for its offspring, then left her body in the hive-like environment.

• From a storytelling angle, her quick, brutal death is used to show how lethal the Upside Down is and to shock the audience without killing one of the core kids.

What happens to Will

• Will is attacked the same night but escapes on his bike, hides, and then is taken into the Upside Down, where he survives for days by staying hidden and using places like “Castle Byers” as shelter.

• When he is finally found, he is cocooned and barely alive, with a similar slug later coming out of his mouth, implying the creature was either using him as a host or had only recently begun the same process it completed with Barb.

• Will is rescued relatively soon after being fully captured, so there is less time for him to be completely drained or consumed the way Barb was.

The main reasons fans infer

• Different timing and circumstances: Barb is alone, injured, and taken directly into the lair, while Will runs, hides, and is only fully caught much later, which delays him reaching the same “end state” Barb does.

• Possible “host” vs “food” role: The slugs from both bodies led to theories that the creature (and later the Upside Down’s larger intelligence) was experimenting with humans as hosts; Barb dies in the process, while Will survives long enough to be rescued and later becomes a key psychic connection to the Upside Down.

• Narrative purpose: Barb’s death shows the stakes and drives Nancy’s story, while Will has to stay alive because his disappearance is the central mystery and his ongoing connection to the Upside Down is crucial for later seasons.

So, the simplest way to put it: Barb dies because the Demogorgon fully uses and discards her, and because the story needs a shocking, permanent loss; Will lives because he evades it longer, is rescued sooner, and is too important to the ongoing plot.