We fans of Demon Slayer: Kimetsu no Yaiba, still remember the shocking start of the series. Tanjiro came back home to find his family slaughtered, and Nezuko turned into a demon. That one moment set the entire story into motion and raised one burning question: Why did Muzan turn Nezuko into a Demon?

This is even more in discussion after the swordsmith village arc ending. As Nezuko is the only demon who survived the sun. Season 1 began with this tragedy, but the answer stretches back to Muzan’s past. From the Heian era to the Swordsmith Village arc. The mystery around Nezuko connects to Muzan’s deepest fear, the sunlight. Let us go through every detail step by step and understand what is his real motive.

Muzan’s Hunt for Sunlight and a “Perfect Demon”

Muzan’s curse started in the Heian era when a doctor treated his terminal illness with an experimental medicine made from the Blue Spider Lily. The cure saved his life but twisted his body, turning him into the first demon. It gave him strength, immortality, and hunger for human flesh. But it also left him with a fatal weakness, sunlight.

The manga explains this in Chapter 127, while the Blue Spider Lily first appears around Chapter 67. After killing the doctor who made the medicine, Muzan lost the full recipe and spent centuries trying to find it again. The problem was brutal: the flower blooms only in daylight, for a few days a year, and sometimes not at all. Muzan could never see it because he could not step into the sun.

His plan became twofold. First, keep hunting the Blue Spider Lily. Second, to create demons with his own blood, hoping one would evolve to survive daylight. He ordered even the Twelve Kizuki to track down the flower, berating them when they failed, like he did with Akaza. Every move came back to one motive: to erase the sun as his weakness and secure true immortality.

Why Did Muzan Turn Nezuko Into a Demon?

As we discussed in the previous section, Muzan’s reason is tied directly to that centuries-old search. After the Blue Spider Lily failed him, he turned to experiments with humans. Only his blood can create demons, and the more blood he gives, the higher the chance of both great power and death. For hundreds of years, Muzan kept infusing humans, waiting for one to show the right mutation against sunlight.

This is where Nezuko enters. Muzan’s attack on the Kamado home was not personal at first. It was another trial to see if his blood could create the perfect demon. He poured a large dose of his blood into Nezuko, far more than most victims could survive. That risk nearly killed her, but also made her one of the rare demons who could withstand this transformation.

Was Kamado’s Home a Random Slaughter?

The anime makes the Kamado tragedy clear in Episode 1, “Cruelty”. Tanjiro came home to find his entire family dead, with Nezuko as the only survivor. Soon after, Urokodaki explained that only one demon had the power to create demons — Muzan Kibutsuji.

The manga shows this in Chapter 11, tying the murder to Muzan himself. What is important is the timing. Muzan’s reaction to Tanjiro’s Hanafuda earrings happens later in Asakusa, in Episode 8, when he first sees Tanjiro. That proves Muzan did not target the family because of the earrings.

Instead, the Kamados became another group of test subjects. Nezuko’s survival with such a large blood dose was unusual, and that is why she grew stronger compared to ordinary demons. The rest of the family died instantly, which fits Muzan’s deadly trial-and-error experiments.

Why Muzan Left Nezuko Behind

When Tanjiro returned home, Muzan was long gone. Nezuko was already transforming, showing signs of the demon blood taking effect. Muzan’s behavior in the series explains this. He never lingers at a scene. In Asakusa, for example, he scratched a passerby with his blood and left immediately. His blood is both a gift and a curse — if the body cannot handle it, the victim dies on the spot.

Leaving Nezuko behind was consistent with this pattern. Muzan likely assumed she would either die or become a mindless demon. He had no need to stay. His top priority was always to move freely in human society under disguises, not to waste time waiting for a subject’s outcome.

Nezuko: A Byproduct, Then a Target

At first, Nezuko was nothing more than another product of Muzan’s reckless experiments. He did not chase her immediately. That changed much later, in the Swordsmith Village arc finale. In Episode 11, “A Connected Bond: Daybreak and First Light” (June 2023), Nezuko stood in direct sunlight and survived.

The scene showed Muzan’s shock as he realized the “perfect demon” now existed. Centuries of searching, failures, and deaths suddenly meant nothing. He no longer needed the Blue Spider Lily. All he needed was Nezuko. From then on, she became Muzan’s prime target, not as a failed experiment but as the key to true immortality.

In the End

The mystery of why Muzan turned Nezuko into a demon has fascinated fans from the first episode. It connects her tragedy to Muzan’s ancient curse, the Blue Spider Lily, and his endless hunt for sunlight. What started as another experiment ended with Nezuko becoming the single greatest threat to Muzan’s power.

That twist is what makes this topic so loved in the fandom. We fans still debate the meaning of her transformation, and it remains one of the most dramatic turns in Demon Slayer. What do you think — was Nezuko’s survival fate or an accident? Share your thoughts in the comments below.