MengXiang:
I don't quite understand or buy why does TH or FFYY want to kill XY? How was that plan sound or meant to work?
The novel plotline makes way more sense than the drama plotline. But there is some logic to the drama plotline.
TSH had a long-standing grudge against TSJ. TSJ also exposed TSH and FFYY's affair, which caused both TSH and FFYY to lose their positions and to undergo punishment. TSH was only exiled to Haoling and prevented from having any power. FFYY endured great pain, was disfigured, and turned into a barely-alive person. In theory, both TSH and FFYY should want revenge against TSJ for that. XY seduced TSH in order to trick TSH and FFYY / to drive a wedge betweeen them that led to the two of them exposing their affair, making it possible for TSJ to collect the evidence he needed to convince the clan elders. In theory, both TSH and FFYY would want revenge against XY for that. And while TSJ (FFYY's husband) was not interested in FFYY and tried to end their engagement until the pregnancy forced him to proceed with the wedding, he was in love with XY and, in front of TSH and FFYY, did whatever XY asked without worrying about his dignity. Even though she didn't love TSJ, that likely hurt FFYY's pride and also would have made her want to take revenge against XY.
But, IMO, the drama probably isn't trying to convince us that TSH and FFYY would have decided to try to kill XY on their own. For the drama, I assumed that Xing Yue knew how badly TSH wanted to kill TSJ and used that for her own purpose. She probably agreed to give TSH the men he needed to kill TSJ if, and only if, TSH agreed to make sure that XY died. TSH was willing to agree to that condition (which seems pretty dumb if he planned to live on after killing TSJ and XY, as he should have anticipated that CX would destroy anyone, other than Xing Yue, involved in killing XY) because that was the only way he could take TSJ out and he also hated XY anyway. Xing Yue and TSH both thought that FFYY would be on board with that plan too. But in the end FFYY cared more about getting revenge against TSH, so Xing Yue and TSH's plan failed.
Edit: I should probably say that Xing Yue's plan failed because of FFYY. TSH's plan failed because of FFYY (who ended up killing TSH and prevented him from killing TSJ in a different, more absolute way), XY (who gave TSJ a life-saving pill that she made from her own blood), XL (who prevented sea monsters from eating TSJ's body), CX (who gave the order for people to continue searching for TSJ), and Lirong Chang (who ended up finding TSJ's body).