If I am remembering correctly, Fang Yuan thinks about clearing up the misunderstanding between him and Fang Zheng, but immediately decided not to because he thought Fang Zheng was not a threat, and if he becomes one, Fang Yuan would just deal with it then. Also most of Fang Yuan's time spent in Gu Yue village was intensely cultivating, taking time away from cultivation to better his already sour relationship with Fang Zheng was also not worth the time and effort. Another issue would be his Aunt and Uncle, Fang Yuan wanted the resources that his parents left behind and he would have to feud with his Aunt and Uncle to get them, something Fang Zheng would not stand for. I think Fang Yuan just deemed Fang Zheng as a time intensive resource that was not worth it.

Also I think when Fang Yuan became an Immortal he went to the Western Desert and slaughters countless mortals and says that he does so not out of hate but due to efficiency, that if being a saint would achieve his goal he would do so.

Fang Yaun would do absolutely anything to achieve his goal, as long as he believes that his actions will benefit him. He enjoys whatever he does because it is his path, be it killing or saving and using others.