Nezuko becomes a demon after Muzan Kibutsuji massacres the Kamado family and injects his blood into her, turning her into one of his many “experiments” to create a demon that can withstand sunlight.

What Actually Happens to Nezuko

  • Muzan attacks the Kamado family while Tanjiro is away selling charcoal, killing everyone except Nezuko.
  • Instead of simply killing her, he forces his blood into her body, which is how demons are created in Demon Slayer.
  • The process works: Nezuko “dies” as a human and revives as a demon, found by Tanjiro when he returns home.

In-universe, turning into a demon is a deliberate act; Muzan is essentially injecting a huge amount of his blood and seeing who survives the transformation.

Why Muzan Turned Her (Motivation)

From what the story and later commentary imply:

  • Muzan has one big goal: become the perfect demon by overcoming his weakness to the sun.
  • He repeatedly creates demons hoping one of them will naturally develop immunity to sunlight; every new demon is basically a test subject.
  • The Kamado family is just one of many victims he chooses to “try out” a large injection of blood on, to see if anyone survives and gains a special ability.

There is no clear canon scene where Muzan says “I chose Nezuko for this specific reason”; most official explanations treat her as another experiment who happened to survive and later develop a unique resistance to sunlight.

How Her Demon State Is Different

Once she becomes a demon, Nezuko is unusual compared with others:

  • She can fight and grow stronger without consuming human flesh or blood, which is extremely rare among demons.
  • It’s implied her demon transformation was never fully completed, which leaves room for her to stay emotionally attached to Tanjiro and later return to humanity.
  • Later in the story, she awakens stronger “forms” in battle, but still retains a core attachment to her human memories and family.

Many fans and forum discussions point out that the series never fully dissects the “why exactly Nezuko is so special,” beyond her being a standout, almost “chosen” demon with exceptional willpower and a unique body.

Quick Forum-Style Take

“How did Nezuko become a demon?”
She was turned by Muzan when he slaughtered her family, using her as one more test to create a demon that could survive sunlight. The twist is that this “random” experiment ended up producing the one demon who not only resists the sun, but also keeps enough humanity to fight against him.

TL;DR: Nezuko became a demon because Muzan attacked the Kamado family and injected his blood into her, hoping to create a powerful, possibly sun- resistant demon; she survived the process and turned into a uniquely “incomplete” demon who could grow stronger without eating humans and later resist sunlight.

Information gathered from public forums or data available on the internet and portrayed here.