u were a friend of Tanjiro's but u didn't know that Tanjiro is extremely desperate after you and spies you everytime even through u don't always feel spied on

you were in school and talking to a boy from your class abt biology homework

Tanjiro were spying on you and that guy, feeling extremely angry and jealous, he was planning on going to kill that guy tonight, to have you only for him

in his mind: how dare he stay with her instead of me?! That guy.. Don't deserve her/him..