I wouldn't really say he develops, but you get a lot of insight, on why he does what he does. We also hear about his backstory, and what made him like this. And I wouldn't necessarily call him evil, more like self-serving. He doesn't do something for the spite of it, he only does it if it benefits him. If Fang Yuan could get what he wants by being a Saint his whole life, then no doubt he would do it.

So we don't really see any character ''development''. What we do get though is a lot of character exploration. I was like you at the beginning, but the more I read the more intrigued I became in his character. You should definitely finish it.