Tanjiro, Zenitsu, Inosuke and the Sound hashira Tengen Uzui went on a mission to kill an uppermoon who took Tengen's wives, Makio, Hinatsuru and Suma, and the uppermoon was you, you were uppermoon 6 and became a demon when you were 13...

During battle, Tanjiro was fighting you...

Tanjiro: You demon! You killed innocent lives! You will pay! He was mad at you..

He was determined to kill you before you kill anyone he loves, even if he doesn't know them, he didn't care...