The Transmigrate Into a Book Ending Completion Plan

I'm a modern-day corporate drone. One night, I went looking for a novel to read, only to discover that the author had abandoned it! Fuming, I rolled my eyes and somehow ended up inside that unfinished novel. I even came with a System about as advanced as Windows 98. And I have to complete the System's ultimate mission: "Finish the ending!" Well then, I'll just have to improvise. The protagonists all have their own halos. So surely I can quietly swoop in and claim a little for myself, right? Along the way, I'll cultivate immortality, take on a disciple, witness iconic scenes, and enjoy the gossip. What a carefree life. One little disciple, burdened by a blood feud and sworn to seek revenge, whom I intercepted and adopted. One aloof immortal cultivator who believes the Heavenly Dao cannot be defied, and that demons, humans, and immortals cannot coexist under the same sky. One Demon Venerable, once the supreme ruler of the demon realm, who insists on causing chaos in the cultivation world and explodes into a corpse at the slightest disagreement. One spirit beast whose clan was also wiped out, always on the verge of going berserk and unable to tell friend from foe once it does. And one mysterious, eerie spirit... Just as I thought everyone except my own disciple was abnormal, I discovered that it was all a facade! Every last one of them is rotten to the core! This isn't just a case of destroying heaven and earth. They're going to tear me into pieces! Why did nobody tell me that the more I tried to fix the story, the more wrong things would get? And why are there even characters turning evil? I thought I was treating all this like a "game": fight monsters, level up, and keep going until I cleared it. Little did I know, I was already in too deep...