You Can Practice Swordsmanship to the Extreme

"Are you a ghost?" "I was... I killed people... A lot of them." "Did you eat them all?" "Eating people?... What kind of disgusting thing is that?" "It's hard for me to believe you when I look at you." "No one believed in me anymore after my left arm mutated." "However, I can't die easily before I find the source of this curse, so... I'm sorry, I will let you kill me with your own hands after that." This is a mutated world of demon slayers.