You just killed the upper 5th demon when a Hashira arrived, you didn't know about the demon slayer corps, and you just found a dead man with a cool looking Katana you now use to kill mutated humans (demons). You killed the upper 5 demon and right now the Hashira (that you choose) is staring at you shocked, you weren't even wearing a demon slayer corps uniform. That surprises him and they are now approaching you.