The primary antagonist of Demon Slayer: Kimetsu no Yaiba, Muzan Kibutsuji is the Demon King and the progenitor of all the demons. He lived for over a thousand years and formed the 12 Kizuki, a group of strongest demons who serve under him as henchmen.
Muzan also creates demons every now and then, like he annihilated the Kamado family and, in the process, turned Nezuko into a demon. Being the most powerful demon, it still puzzles the fans about this specific action, where he turns ordinary humans into demons without devouring them instead.
Why did Muzan create Demons in Demon Slayer?
Muzan Kibutsuji is the deadliest force in Demon Slayer, as he has abilities like altering his age, shapeshifting, and unique blood capable of making formidable demons.
Despite being the most powerful being, he considers himself incomplete because he is still not capable of overcoming the Sun. Muzan wants to become the perfect creation where he becomes flawless and can walk in broad daylight.
This flower was an essential ingredient behind turning him into a demon in the first place, although it was meant to cure his terminal illness. He desperately needs this unique flower to become immune to the Sun.
Another thing that could make him immune to the Sun is his own creations. He hopes that a demon will come along conquering the Sun one fated day, so he can devour the demon completely and assimilate its powers.
So there are many reasons why he creates demons, but the most specific reason would be gambling his luck to overcome his weakness by finding the perfect vessel who can conquer the one thing that he was not capable of.
Once he accomplishes his dreams, he will be able to spread his tyranny even in the daytime.