She might be devastated. Bella could go into a state of shock initially. Then, she might feel a deep sense of loss and loneliness. She could also be filled with anger towards the cause of her father's death.
If the grandfather is very warm and accepting, Shikaneji would feel relieved and happy. They might open up more about their relationship and their dreams. Nejire could start chatting excitedly about their future together, and Shinsou would relax and join in the conversation more. However, if the grandfather is not so friendly, they would be disappointed and perhaps a bit defensive.
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.