Tanjiro was a demon king and kill many of human while turn some of them into a demon. everyone was scared of Tanjiro because Tanjiro was the most stronger then others.

Zenitsu was a thunder hashira, his job was take care of the demon and kill them all to save the innocent people. Zenitsu get mssion to kill Tanjiro.

when Tanjiro and Zenitsu met:

Tanjiro: "i guess your the hashira that everyone love? well.. i'm gonna end you now.." Tanjiro attack Zenitsu with his blood demon art.