While it is not explained in the anime why he did so, Muzan is shown to be living a normal life as a human. Not only that, he's married a woman and has a child with her.
When Demon Slayer's Tanjiro met Muzan in Tokyo, Muzan had a human wife and daughter with him. There's a chance he genuinely cared for them. In the story of Demon Slayer, demons are supernatural creatures of the night who consume human flesh to survive. They are weak to sunlight, making them a variation of vampires.
Anne was originally planned to be Muzan`s older sister, who died by the hand of a Demon Slayer but that idea was scrapped, as the author knew practically nothing about Muzan`s early life. Instead, Anne became his wife, by which he had two children.
Simple. He killed the real father and replaced him.
This allowed him to hide from the Demon Slayer Corps while he carried out his experiments in an attempt to create a demon without any weaknesses. Therefore it is quite evident that Muzan doesn't love his family. They are merely tools to achieve his goal of true immortality and wreak havoc in the years to come.
Curse of Kibutsuji
This curse besotted any who partake of Muzan's blood must be careful. The curse prevents Muzan's name from being spoken aloud, and any who do will meet a horrible demise. One demon, Susamaru, faced this reality when battling Tamayo.
Even 400 years later, Muzan's fear of Yoriichi remained fully intact, as simply laying eyes on Tanjiro's hanafuda earrings caused him to recall their clash.
Yoriichi was incredibly proficient in his Sun Breathing, being able to take down demons effortlessly and even overwhelm the Demon King and strongest Upper Rank.
Muzan has had more than one family over the course of his millenia-long life. This includes the birth family he had before he became a demon. In addition to them, Muzan took five wives.
For those who have watched only the first season of the anime, they will be surprised to know that Muzan turns into a woman in the second season. He keeps transforming in order to hide his true identity, and he's even known to change into an 11-year-old child to hide from Demon slayers.
It is revealed that Muzan's true fear of dying and immense desire to live and survive was due to him being born "dead," where his heart would constantly stop in his mother's womb and was assumed stillborn until he fought to breathe before his cremation.
Don't go! DON'T LEAVE ME HERE! Muzan speaks his final words as he loses control over Tanjiro.
3) Gyutaro. Gyutaro is another one of Muzan's strongest bastions. However, he took a twisted approach to his dealings. Since Muzan favored Gyutaro so much, he chose to incorporate the latter's sister Daki as well.
Haruka Shiraishi: Muzan's Daughter.
While it is not explained in the anime why he did so, Muzan is shown to be living a normal life as a human. Not only that, he's married a woman and has a child with her.
Hoshiko Kibutsuji. The first female demon [of a different orgin], and the younger sister to Muzan Kibutsuji. She loves her brother and was the one who had attacked that man who wore the hanafuda earrings.
Fun fact: The reason why Enmu was able to say Muzan's name out loud without dying is because they're married!
Kie is a warm and loving person; as the mother of 6 children, she is always seen displaying a very affectionate and caring personality.
The explosion, which was enhanced by spike traps, manages to injure Muzan. Before he is able to regenerate fully, Tamayo appears and uses her Blood Demon Art to keep him in place, while using a drug that can turn him into human. Using this opportunity, Gyomei Himejima appears and destroys Muzan's head.
Yoriichi lived until around 85 years, so: 1432 + 85 = 1517, exactly 400 years prior to when Kokushibo remembered and died.
The first demon that is said to have existed was Muzan Kibutsuji. The one who turned him into a demon was a generous doctor from the Heian Period, who wanted to save Muzan from death since, at the time, he was diagnosed with a disease which would kill him before he turned twenty.
Abilities. Demon Body: Unique from other demons, he possesses seven hearts, and five brains. These are responsible for his powerful vitality, should any of them be harmed, he will be severely weakened.
Since he didn't know anything about the Blue Spider Lily medicine, he needed others to find the plant for him by happenstance. This is the general reason why Muzan turned Nezuko into a demon. More specifically though, was the fact that Nezuko was of Kamado blood.
However, Tanjiro is not a descendant of Yoriichi Tsukiguni. Tanjiro is the descendant of someone who knew Yoriichi personally and was a good friend of his as well. Sumiyoshi was rescued by Yoriichi when a demon threatened his life. Since then Yoriichi often met him and they became close acquaintances.