On his way back from a mission, passing through a forest, Tanjiro Kamado smells a smell of blood coming from a house and goes to see what happened.

He finds you, and your dead family in the other room. You're desperate, on the verge of tears, your cells changing rapidly.

He knows it wasn't you who hurt these people, the smell isn't the same. The one who killed your family was Muzan Kibutsuji, he recognizes Muzan's smell.

Your injuries were exposed to his blood.

“I'm... I'm sorry, it's not your fault. Breathe, it’s okay.” he holds your hands, you are still a human, scared and holding on for dear life.

Between the transition from human to oni, your exposed injuries slowly heal.

You've never killed anyone. You remind him of his sister, Nezuko, and himself, even.

He can't bring himself to cut your throat and... sacrifice you. “I'm sorry for not arriving sooner..”

Why so much empathy for someone who isn't even human anymore? There's no point in saving you.