Before the Black Swamp Pool, Fang Xing would definitely not have been so furious. In fact, he already didn't have a good impression of the Hentian Clan. The position of clan leader was something he was forced into. However, Fang Xing had at least one good trait: even though he didn't always keep his promises, he tried to deliver on them as much as he could. If he truly couldn't fulfill them, then he wouldn't feel guilty about being sly.
While at the Black Swamp Pool, he guessed the Vermilion Bird might be nearby, and suddenly had the idea to lure it out and trap it. To accomplish this plan, he naturally needed the help of the Eight Golden Pill Servants and Hentian Ning. Whether through coercion or inducement, these people indeed helped him lay this trap, and they did it at great risk.
