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be careful ! The following contains major spoilers for Demon: Slayer: Kimetsu no Yaiba, by Koyoharu Gotouge.
Demon Slayer: Kimetsu no Yaiba begins like any great story: with the brutal slaughter of an entire family made up mostly of children. When Tanjiro Kamado returns home to find most of his family dead and his sister Nezuko turned into a demon, he makes it his mission to go after the demon responsible.
A few years later, after joining the Demon Slayer Corps, Tanjirou meets Muzan Kibutsuji while visiting Asakusa City. There was Tanjirou confirming with the scent of Muzan that he was the demon who massacred the Kamado family. But why did the origin, ancestor, and demon king decide to slaughter a family living in the mountains?
Muzan Kibutsuji, the demon slayer, has a huge weakness!

Muzan Kibutsuji, despite being an evil demon who lived 1,000 years and killed thousands of people, has a very realistic motive: He doesn’t want to die. Muzan was dying of illness when a doctor in Heian era Japan used the mysterious blue spider lily to turn Muzan into the first demon. However, Muzan soon realizes that even though he has gained incredible powers, he will die if he is exposed to sunlight. Muzan found it humiliating to have to stay home during the day and spent the rest of his long life searching for a way to become the perfect person, immune to death.
Besides the sun, Muzan’s biggest fear is the same sun, the original and most powerful breathing technique used by Demon Slayers to fight demons. Yoriichi Tsujikuni, the first swordsman to use breathing techniques, nearly kills Muzan with his sun breath, leaving Muzan damaged both physically and psychologically. Muzan went to great lengths to ensure Sunbreath was eliminated. Associated with Kokushibo, one of the Twelve High-ranking Kizuki and Yoriichi’s brother, Muzan and his most powerful demon roamed Japan, killing everyone who heard of Sunbreath. The only people she missed were the ancestors of the Muzan family who were massacred at the beginning of Demon Slayer: Kamados.
Why did Muzan kill Tanjiro’s family?

For a while, fans suspected that the reason Muzan killed Kamados was because he somehow knew they had died in the same sun. Like Hinokami Kagura. Luckily for Muzan, Tanjurou Kamado, Tanjirou and Nezuko’s father passed away because Tanjurou was a true master of Hinokami Kagura. However, a flashback from Nezuko’s perspective regarding the attack in the manga’s final arc, “Sunrise Countdown”, He confirms that Muzan’s motive for slaughtering the Kamado family had nothing to do with Breath of the Sun, and that it was ultimately a successful scheme.
Muzan Kibutsuji hates demons even after creating every single one of them. It turns out that the only reason Muzan continued to create demons, despite not wanting to, was hoping one of them would develop the ability to resist sunlight. When Muzan killed the Kamado family, he injected each of them with a massive dose of his blood, hoping that the ultimate power they would gain would spawn a demon that could walk on the sun. Nezuko is the only one who survives the amount of blood injected from Muzan, and at the end of the “Village of Swords” arc in the manga, Nezuko survived being exposed to the sun’s deadly rays.
Demon Slayer manga has never been confirmed Why choose Muzan Kamado in particular?. Perhaps he just thought that no one would notice Kamados’ disappearance, since he preferred to stay out of sight anyway. By creating a demon that can survive exposure to sunlight, Muzan would only have to consume that demon to gain this ability for himself. Muzan’s experiment on Nezuko worked just as he had hoped, but unfortunately for him, he chose the wrong family to pursue, as Tanjirou joined the Demon Slayer Corps, which eventually ended. This led to Muzan’s defeat and death.