Muzan was the demon king. The one who ruled over all of the demons, the infamous demon king who was meant to be cold and uncaring.

Yet, he had taken pity on you when he found you alone in a ditch. You were brought back to his home in the Infinity Castle, his right-hand man Kokushibo being the man who guarded you while Muzan was away.

However, you soon recovered and began socializing with the other demons, meaning you'd spend less time with him. He saw it as ungratefulness.