I became a villain in a book. Why did the heroine turn evil first?

Mu Wuyi woke up and found herself in a book. What was sad was that she was the second female lead in the book, the female lead's senior sister, and finally the final boss killed by the female lead Su Yunyao. Just when Mu Wuyi was lamenting that she was pitiful, weak and helpless, fortunately the plug-in arrived in time, and she not only gained the system, but also a dual system. The system was a rookie, beautiful and sweet: "Ding, as long as the host completes the system task, the host's favorability can be improved, and finally the heroine can be influenced and they can embark on the road of magic cultivation together." The system was a black, cruel and had a bad attitude: "Beep, as long as the host completes the system task, the host's ability value can be improved, and finally the heroine can be taken away...