Because once Yingsuiyue exchanged blows with Zhixing first, it was very obvious that with Yingsuiyue's current strength, wanting to knock him out was simply too easy. It only took three to five moves. But what Long Yueling wanted was not for Yingsuiyue to faint, but to kill him.
If Qin Zhixing did not have the "Pursuing Shadow" in her hands, Long Yueling would never be afraid of Qin Zhixing, nor would he care about her life and death.
Since Yingsuiyue's feelings towards her were real, and since Yingsuiyue simply could not contain Qin Zhixing in her heart, why should she bother with him? Wasn't it lowering its status?
But... The result would be very different if he followed along like a shadow.
Just this pill alone was enough to turn the situation around. The problems of internal strife, the problems that Yingsuiyue's heart was preoccupied with... Everything that had just been resolved would instead sink into an even more difficult situation.