How Did Muzan Become a Demon?

Additionally, Muzan instinctively realized that he would die if he were ever to be exposed to sunlight. Whatever medicine the physician had crafted using the Blue Spider Lily had effectively granted Muzan immortality, transforming him into the world's first demon.

Who turned Muzan into a demon?

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.

Why did Muzan become evil?

However, as the time went on, Muzan eventually realized that he had become better physically and the doctor's medicine was indeed working, but he was unable to walk in the sun anymore. Muzan also realized that he had cravings for human flesh as sustenance; Muzan had now become a man-eating Demon.