Demon Slayer: You call paying to get stronger "lightning"?

[System pays for life][Exhilarating combat][Thunderous][Growth]When Yasui Ryosuke is transported to the world of Demon Slayer, he is devastated.Good news: He is familiar with the original novel and knows the plot.Bad news: The original owner was a sickly person who was not long for life, and the only cheat was the double-dealing system that required spending a lot of time and effort to become stronger!"You mean killing demons grants you lifespan, which can then be used to enhance your breathing technique and sword skills?"Yes, that's right~How much longer can I live?[Congenital liver failure, at most one year, haha~]"Fuck your sister!"Yasui Ryojie was angry.What kind of good system would make a cancer-stricken host risk their life?Knowing the original plot, Ryosuke would never willingly seek his own death by joining the scoundrel Muzan.If that's the case, then kill!Slaughter them until the heavens and earth are darkened, and annihilate all the evil spirits in the world!……The evil spirit: "Don't ghosts have the right to live? Do ghosts deserve to die?!"Tanjiro pondered deeply, while Zenitsu remained silent.Ryosuke drew his sword: "Nonsense! Do you think I'm the one who dies if you don't?! Go talk to my Nichirin Blade!"……Nightmare: "Fall into a beautiful dream."Ryosuke: "Kill! Kill! Kill! All evil spirits deserve to die! Cut them into eight pieces! Chop them into mincemeat!"Nightmare carried the train and fled overnight.……Akaza: "Ryosuke! Become a demon, Ryosuke!"Ryosuke Yasui's eyes widened like saucers.Dude, are you drunk?!……Black Death Eyes: "Throughout history, you are the first person to have mastered the Breath of Thunder to its ultimate level."Ryosuke: "What are you babbling about? System, give me some points! Damn it!"……Muzan: "Is killing me all you think about?!"Ryosuke: "Hehe~ Demon King~ Huge experience pack~ Hehe~"