It is essential to understand that Muzan is an evil demon who loves to kill human beings. Sometimes, he might have a reason, but it is not always required. For example, he might kill someone just for fun because killings pleasure him.
In the case of Tanjiro’s family, however, he had some reasons to kill them. Yoriichi was, for example, one of his most significant and dangerous enemies. He defeated him in one battle that occurred in the distant past. Wildly few demon slayers have succeeded in destroying Muzan, and Yoriichi was one of them. That’s why Muzan has never forgotten him.
He never succeeded in avenging Yoriichi because he was too strong for him. However, he found a way to fight back against Yoriichi. The decision was to kill his family for that purpose, and Tanjiro was a direct descendant of Yoriichi. That’s why Muzan decided to kill them all, but he did not complete his mission entirely. It is because Tanjiro survived the attack and is now seeking revenge.
Muzan was an ancient demon. He has been active for hundreds of years and has killed numerous people in his evil endeavors. Still, he was not always the most powerful individual in the world. There have been some other demons who have had the same or even greater strength than Muzan.
Some of those beings have managed to defeat him in battles. Muzan was usually destroyed at those moments. However, he somehow succeeded in surviving.

Why Did Muzan Attack Tanjiro’s Family?
As mentioned, Muzan attacked Tanjiro’s family as a sign of revenge for the defeat he had to suffer from their ancestor Yoriichi. However, he was not capable of fighting directly against him. That demon slayer was too strong for Muzan. Muzan did not forget the words and pain he suffered at those moments and remembered Yoriich for a lifetime.
Revenge was his primary mission, but he couldn’t fight against Yoriichi again. That’s why he waited for a perfect moment to take revenge. However, Yoriichi was already quite old, so he died at 85. After his death, Muzan could not take revenge directly against him, but he still could not forget his defeat. That’s why he succeeded in attacking and killing his later generations.
Fear is another reason why Muzan decided to kill Tanjiro’s family. He was aware of the enormous strength of his ancestry, so he worried that someone might get the same powers as Yoriichi. His opinion regarding inheritable characteristics was correct, but it does not justify his attack.
Why Does Muzan Have A Family?
There are indeed some practical reasons why Muzan has a family. He was typically known as an evil and cruel demon; however, no one knew what he looked like. That caused a lot of trouble for Tanjiro while he was searching for him. While trying to imagine an ancient and nasty demon, you will probably think of some monster. It is challenging for such a creature to live in a community with average citizens, so you would probably not go to find him in any town or village.
Maybe deep in some jungle or high in the mountains where no one can find him. However, such an opinion would be wrong because Muzan lived in a city like any other ordinary citizen. And that’s the most important reason he had a family with him. It was used as a part of his camouflage.
No one would think anything wrong about an ordinary person with an average family. His appearance was excellent, and he looked like a regular person when Tanjiro found him. His family looked the same, so Tanjiro was surprised by his discovery. No chance anyone in the city would suppose anything negative about these people.
Muzan seemed like a young boy when he was 1,000 years old. He could use his family as a shelter. He could move from one place to another without being noticed by other people. He managed to attack and kill some people while staying undiscovered. Muzan did not care much about his family from an emotional point of view. He just used them for his own goals.

Why Did Muzan Turn Nezuko?
Nezuko is Tanjiro’s sister and a part of the Kamado family. That’s because the monster saw potential in her, so he decided to spare her life. Still, she did not stay safe and without consequences because Muzan turned her into a demon.
Except for her, only Tanjiro survived Muzan’s attack because he was not present. Nezuko was the youngest girl in the family and an adorable kid. Even the cruel demon Muzan had some positive feelings for her.
He initially wanted to turn some of the Kamado family members, but none had the power to survive the transformation. Only Nezuko could do so, signifying big success in her demon’s career.
She still looks like a regular human being, and no significant differences are comparing the new and old looks. Nezuko also wears similar clothes, with very few exceptions that disappeared during the transformation. That’s a positive aspect because she will have a chance to live unnoticeably among human beings in towns and cities.
Does Muzan Love His Family?
Unfortunately, Muzan does not love anyone, not even his own family. He uses them as a shelter. There is an ugly story behind the creation of his family. She had another husband, and they loved each other. As a solid and powerful demon, he murders her husband and acts like her new boyfriend.
The women already had kids, but they call Muzan dad, but they are unaware that he does not love them as they love him. Muzan’s family is not also informed about his past. They do not know the lovely man killed their birth father.
This guy is an ancient and evil demon who constantly puts them all at risk. They would probably be terrified if they learned more about his real nature. So they killed numerous people and have often been somewhere nearby during this.