Muzan Kibutsuji did a number of awful things in Demon Slayer, but one of the most intriguing is his murder of Tanjiro’s family. The Demon Slayer: Kimetsu no Yaiba begins like any good story should: with a horrible killing of a whole family, mostly comprised of children. Tanjiro Kamado made it his mission to uncover the demon that had turned his sister Nezuko into a monster after returning home to find most of his loved ones dead. Tanjiro met Muzan Kibutsuji when visiting Asakusa after joining the Demon Slayer Corps a few years later. The fragrance of Muzan verified to Tanjiro that he was the monster who slaughtered the Kamado family.
Despite being a wicked demon who has lived 1,000 years and killed thousands of humans, Muzan Kibutsuji has a reasonable motive: he does not want to die. Muzan was dying of disease when a healer in Heian-era Japan transformed him into the first demon using the magical blue spider flower. Muzan quickly learned, however, that despite his extraordinary talents, he would perish if exposed to sunlight. Muzan found it humiliating to have to stay inside during the day, so he spent the rest of his long life trying to figure out how to become the perfect being immortal.
Apart from the Sun, Muzan’s biggest fear is Sun-Breathing, Demon Slayers’ original and most potent means of combating demons. Muzan suffered physical and psychological consequences after Yoriichi Tsugikuni, the first swordsman to use Breathing Styles, nearly killed him with the Sun breathing. Muzan went to great lengths to make sure the Sun breathing was no longer present. Muzan and his most powerful demon went on a tour of Japan, murdering anybody who had heard of the Sun breathing. Muzan was linked to Kokushibo, Yoriichi’s brother and one of the twelve high-ranking Kizuki. The only people they were missing were Muzan’s ancestors, the Kamados, who Muzan massacred at the opening of Demon Slayer.
For a time, fans suspected Muzan of killing the Kamados because he learned that they transmitted the Breath of Sun as Hinokami Kagura. Tanjuro Kamado, Tanjiro and Nezuko’s father, had already died, which was fortunate for Muzan because Tanjuro was a great Hinokami Kagura master. However, in the manga’s final arc, “Sunrise Countdown,” a flashback from Nezuko’s perspective revealed that Muzan’s purpose for murdering the Kamado family had nothing to do with Sun-Breathing, and that it was ultimately a successful ruse.
Muzan Kibutsuji despises demons because he is the one who created them all. Muzan’s only motivation for generating demons, despite his objections, turned out to be the hope that one of them might gain the capacity to resist the sun’s light. Muzan gave each member of the Kamado family a massive dose of his blood, believing that the sheer power they would get from him would result in the birth of a monster who could walk in the sun. Only Nezuko survived the amount of blood Muzan injected into them, as well as her exposure to the sun’s fatal rays near the manga’s “Swordsmith Village” arc’s conclusion.
Muzan’s choice of Kamados was never addressed in the Demon Slayer manga, thus it’s possible that it was a case of two birds, one stone, with Sun-Breathing gone He may have simply assumed that no one would notice the Kamados’ departure because he preferred to maintain a low profile anyway. Muzan could get the power to survive sunlight exposure by producing a demon that could be consumed. Muzan’s time on Nezuko went exactly as he had anticipated, but he made the mistake of going after the wrong family, as Tanjiro joined the Demon Slayer Corps, resulting in Muzan’s defeat and death.