In Demon Slayer, Kimetsu no Yaiba began with the brutal murder of an entire family, primarily children, as every great tale should. Tanjiro Kamado made it his life's work to find the demon responsible after arriving home to discover most of his family dead and his sister Nezuko transformed into a demon.
Several years later, Tanjiro ran into Muzan Kibutsuji while touring the city of Asakusa. By this time, he had joined the Demon Slayer Corps. By smelling Muzan there, Tanjiro was able to prove that he was the demon responsible for the Kamado family massacre. However, why did the Original, Progenitor, and King of Demons decide to massacre an unnamed family living in the mountains?
Demon Slayer's Muzan Kibutsuji Has a Major Weakness
Muzan Kibutsuji, despite being an evil demon who's lived 1,000 years and murdered thousands of people along the way, has a pretty relatable motivation -- he doesn't want to die. Muzan was dying of illness when a doctor in Japan's Heian era used the mysterious Blue Spider Lily to turn the ailing Muzan into the first demon. However, Muzan quickly realized that although he had gained incredible powers, he would die if he was exposed to sunlight. Muzan found it humiliating to have to stay indoors during the day and spent the rest of his prolonged life searching for a way to become the perfect being immune to death.
Muzan's greatest fear, aside from the sun, is Sun-Breathing, the original and most powerful Breathing Style used by Demon Slayers to battle demons. Yoriichi Tsugikuni, the first swordsman to use Breathing Styles, nearly killed Muzan with Sun-Breathing, leaving Muzan physically and psychologically scarred. Muzan went to great lengths to eliminate Sun-Breathing. Muzan and his most powerful demon traveled around Japan, killing anyone who'd even heard of Sun-Breathing, in collaboration with Kokushibo, Upper-Rank One of the Twelve Kizuki and Yoriichi's brother. The only people they overlooked were the ancestors of the Kamados, the same family Muzan would slaughter at the start of Demon Slayer.
Why Did Muzan Murder Tanjiro's Family In Demon Slayer?
Fans suspected for a time that Muzan had killed the Kamados because he had discovered that they had been passing down Sun-Breathing as the Hinokami Kagura. Tanjuro Kamado, Tanjiro and Nezuko's father, had already died, which was fortunate for Muzan because Tanjuro was a true master of the Hinokami Kagura. However, a flashback from Nezuko's point of view during the attack in the manga's final arc, "Sunrise Countdown," confirmed that Muzan's motivation for slaughtering the Kamado family was unrelated to Sun-Breathing and was ultimately successful.
Muzan Kibutsuji hates demons for having created each and every one of them. As it turned out, the only reason Muzan kept creating demons, even though he didn't want to, was in the hope that one of them would develop the ability to withstand sunlight. When Muzan killed the Kamado family, he injected each of them with a massive dose of his blood, hoping that the sheer power they'd get from him would birth a demon capable of walking in the sun. Nezuko was the only one who survived the amount of blood Muzan gave them, and at the end of the manga's "Swordsmith Village" arc, Nezuko did indeed survive being caught in the sun's lethal rays.
Demon Slayer's manga never confirmed why Muzan chose the Kamados specifically, so perhaps it was a two-birds-one-stone situation with the elimination of Sun-Breathing. Maybe he just assumed no one would notice if the Kamados disappeared, considering his preference for keeping a low profile anyway. By creating a demon who could survive exposure to the sun, Muzan would then only need to consume that demon to gain the ability for himself. Muzan's experiment on Nezuko worked out exactly as he'd hoped, but unfortunately for him, he picked the wrong family to mess with, as Tanjiro joining the Demon Slayer Corps is what ultimately led to Muzan's defeat and death.