Fang Yuan is not "evil"
"Oh he kills people" or "oh he enjoys it" is not a good argument, generally because descriptive nature lacks a prescriptive basis for what should be considered good and evil in the first place, but also because it is both not interesting and incorrect in terms of the setting of the novel.
RI is an eastern novel and is full of different moral stances, this however doesn't mean that a question of "evilness" should be by any means obvious. The idea of "evilness" itself has to be established first and is not obvious despite how pervasive Christian worldview is in our society or here on Reddit.
RI doesn't operate in terms of a Christian framework, instead it is operating on ideas of Buddhism, Confucianism and Taoism which is extremely clear from the start.
I will not go into detail here, because it is actually a good discussion of how the author uncovers topics of principles of these philosophies (intended as a neutral term), but you can easily see how structured hierarchical clan system(and the whole righteousness path philsophy), equality of spirit and the pursuit of eternity with the path of least resistance are central to what Fang Yuan addresses.
Therefore, the question of evilness is terrible and our community should know better than to pose it in such a way, afterall the idea of corruption and perversion of actual principles presented is also apparent and was already discussed properly before.
This is only those posts which I personally enjoyed and which are actually a good discussion of moral systems.
RI and Nietszche
https://www.reddit.com/r/ReverendInsanity/s/XgT8CnLxU5
https://www.reddit.com/r/ReverendInsanity/s/TkXfjTq4kK
RI and Buddhism
https://www.reddit.com/r/IamFangyuan\_/s/SBrQyvUpL3
RI and Taoism.
https://www.reddit.com/r/ReverendInsanity/s/PopQQoqBh2
P.S. I personally would support the idea that the author took the main three Chinese ideas of virtue and made Fang Yuan an inversion of all three, added some Nietszchean ideas of what morality is about and thus fulfilled his goal of creating an ultimate villain, which is still not about Fang Yuan being evil, just a villain. (The novel could even be used as a case study on how Nietszchean ideas would apply to Chinese philosophy. Very unique, isn't it?)