The two end up becoming friendly later in the show, but why did Tanjiro attack Genya after the Final Selection?
Tanjiro decides to attack Genya in order to defend a child. Genya pulls a kid’s hair while demanding a sword, and Tanjiro intervenes. He breaks Genya’s arm after a warning to let go.
Genya and Tanjiro reconnect later in the series during the Swordsmith Village arc.
Tanjiro decides to attack Genya
Chapter #8 takes place 7 days after the Final Selection exam begins. Tanjiro is able to successfully survive the week, defeating the Hand Demon along the way.
Tanjiro is shocked that only 4 of the 20 students who entered the exam have survived. The two mysterious twins who work for the exam welcome back the survivors.
Amazingly, Zenitsu is able to pass the Final Selection. The other survivors include Kanao and Genya. The last of the bunch abruptly demands to be given his katana.
Ignoring the request, the twins say that they’ll take each survivor’s measurements for their Corps uniforms. Each new slayer will have their rank engraved. They are the bottom rank, known as Mizunoto.
The twins explain that today will allow everyone to choose an ore for their swords. It will then take up to two weeks for the katanas to be finished.
Each new slayer is then matched with a Kasugai crow. The crows are generally used for communication, the twins explain.
Genya becomes angered
Suddenly, Genya smacks away the crow on his shoulder. He grabs one of the twins by their hair, demanding a color-changing Demon Slayer Corps katana immediately.
Tanjiro grabs Genya’s arm. He demands that if Genya does not let the child go, he’ll break the arm. Still resisting, Tanjiro tightens his grip and breaks a bone. Later, Tanjiro is confronted about this event after seeing Genya pull out his teeth. Around this same time, Tanjiro learns why Mitsuri decides to become a Demon Slayer, which reveals more witnesses to Genya’s bad attitude.
Genya recoils back in pain but remains quiet for the remainder of the entrance ceremony.
So that’s the answer. Tanjiro decides to attack Genya in order to defend a child. Genya pulls a child’s hair because he demands a sword, and Tanjiro intervenes to save the kid. He breaks Genya’s arm during the confrontation.