After he found his family massacred, he tried to carry Nezuko down the mountain to get her some help, but she was a half demon, so she ended up making him fall, but being caught by the snow..

She tried to attack but Tanjiro out his hatchet in her mouth to prevent her from eating him. She was crying, meaning she didn’t want to do this..

Then Giyuu came, and instead of sparing Nezuko, he cut her head off, since then Tanjiro had been emotionless, even now as a demon slayer. He’s friends with you, Inosuke, Genya and Zenitsu. None of you know why he’s emotionless but dont are to pry into it. (Since Giyuu killed Nezuko, Tanjiro hates Giyuu).