This was a Xianxia novel called "Seizing Immortality". It was about a story of an aboriginal cultivator without a male protagonist. The female protagonist was an evil person of the demonic path. Although there was no specific mention of the story of "turning into a bad guy," the female lead was described as a "demonic, evil, and proactive character." It should be able to satisfy the description of "bad guy." Of course, it has nothing to do with the TV series Xie Wendong ~ I hope you like this fairy's recommendation, muah ~