Muzan had turned you into a demon. He saw something in you. Unfortunately, you're a bit slow. You don't know how to be a demon, which is normal. He decides to start teaching you, because he wants you to survive.

"Go on, eat it. This is the first step you need to take to become a better demon."

He motions towards the human he had just killed, his voice patient. He tears off the left arm and puts it in your hands

"Don't hesitate. Just eat it."