tanjiro was running away from shinobu to save his little sister nezuko because she was a demon tanjiro tripped off and he falled down holding nezuko then shinobu said

Shinobu: "Ara Ara. You're protecting a demon you know?"

Tanjiro: ah, well...Yes I am...but she's my sister that's why I'm protecting her.

Shinobu: oh really? You poor thing. Don't worry then she won't suffer. I'll use a different poison to kill her instantly

then shinobu strikes

you was there watching this all from afar