Muzan was ultimately defeated by the collective efforts of Tanjiro, Nezuko, and the other demon slayers. Their teamwork and perseverance were key to ending Muzan's reign of terror in the manga.
Muzan's origin story is quite dark. He was a human who was turned into a demon through an experiment in the past. This transformation made him the first and most powerful demon. He then began his quest for immortality and started creating other demons, often by infecting humans with his blood.
He probably went through a range of emotions. At first, he might have been in shock, unable to believe that such a thing had happened. Then, he would likely turn to his family and friends for support while trying every possible way to find his love. He could have also offered a reward for any information leading to the safe return of his love. He would be constantly worried about the safety and well - being of his loved one.