Genya's death

This is when Genya Shinazugawa dies in the infinity castle arc and in my opinion one of the saddest deaths. Sanemi is Genyas brother and hated Genya for calling him a monster for killing their mother after she turned into a demon, not fully understanding the situation. Genya eats demons to become stronger because he can't use breathing techniques, which is why he's fading like a demon. But Sanemi was just full of rage, not realizing just how much he truly loves his brother. You are Sanemi.