One of the most pressing questions among fans is why did Kibutsuji Muzan kill Tanjiro’s family? The answer, apparently, is revenge.
The most common and logical reason for Muzan killing Tanjiro’s family is revenge. It is possible that Muzan has either intentionally or accidentally found the bearer of the legendary hanafuda earrings and has gone to the mountains to personally eliminate the last of the bearers of those earrings.
As shown in Chapters 13 and 14 of the manga or Episodes 7 and 8 of the anime, Muzan was visibly agitated when Tanjiro removed his scarf to gag the bystander turned into a demon. The only difference is in the sequence but either way, when Muzan saw the kid with the hanafuda earrings, he remembered the night he almost died in the hands of a demon slayer with the same accessories plus a red sword.
This scene both supports and casts doubt on the revenge theory since Muzan does have a history with the hanafuda earring. However, his anger and flashback reveal that it was his first time seeing it again after a long time.
Why Did Muzan Turn Nezuko Into A Demon?
First of all, Kibutsuji Muzan definitely killed the Kamado family and turned Nezuko into a demon. Urokodaki Sakonji himself confirms it in a flashback on Chapter 11 of the manga and Episode 6 of the anime, saying that only Muzan has the ability to turn other people into demons.
Still, this relates to Nezuko belonging to the Kamado family. It is possible that as a part of his revenge, Muzan turned Nezuko into a demon to dispose of any family member who was not around when he attacked – Chasing Anime’s theory
Nevertheless, it is important to point out that members of the Kamado Family are not direct descendants of Yoriichi Tsugikuni, the Sun Breathing user. The Kamado family comes from Sumiyoshi, a close friend of Yoriichi Tsugikuni who presumably learned the basics of the Sun Breath and had it passed down through the Kamado family as a ceremonial dance.
It’s highly possible since, in the 99th Chapter of the manga, Sumiyoshi asks to do something for Yoriichi as a sign of gratitude for saving him from a demon back then. In his words, Sumiyoshi asks “At least allow me to pass your story to the next generation.”
The next question is, why was Nezuko turned into a demon, and not anyone else in the family?
Why Did He Spare Nezuko?
Using the source material, we can safely come to the conclusion that Muzan sparing Nezuko at first was intentional. The progenitor of all demons has the ability to sense the presence of humans. He can also see and read the thoughts of the demons he created. Moreover, Muzan has also displayed the ability to kill the demons even without direct contact.
Also, it should be noted that demon slayers, retain their breath abilities once they turn into demons. Kaigaku (Upper Moon Six) still uses his Breath of Lightning style against Zenitsu. Even Kokushibo (Upper Moon One) uses his Breath of the Moon in the fight against the pillars.
Muzan spared Nezuko most likely because she expects something from turning a member of the Kamado family. True enough, Nezuko grew and transformed over the series. She demonstrated her Blood Demon Art during the fight with Rui. Nezuko then did her Demon Transformation in the fight against Daki. Lastly, she revealed her resistance to sunlight after the fight with Hantengu.
Nezuko, based on Muzan’s understanding, has some relation to the user of the Breath of the Sun Style. She definitely has a role to play in Muzan’s quest in becoming the perfect being. We know that Muzan is after two things: conquering the sun and finding the elusive blue spider lily. A demon coming from a clan related to Yoriichi has the biggest chance of succeeding and he already turned Nezuko. As for his other mission, he and his minions have been looking all over Japan for the rare flower…
…which brings us to the next question:
When and Why Does Muzan Look Like a Girl
Kibutsuji Muzan in his female disguise first appeared in Chapter 51 of the manga and Episode 23 of the anime. Though not directly explained, it makes sense that it serves the same purpose as his other disguises. Muzan also has his original human form and his child form. He uses his shapeshifting abilities to disguise himself and blend with humans. Through this, he can roam freely and even conduct research regarding the fabled Blue Spider Lily.
Aside from this, Muzan has also been seen in other forms. The most common is the pale-skinned young looking male, probably in late 20s or early 30, with curly black hair. When Tanjiro found him in the middle of the city streets, he was apparently with his wife and daughter.