It was Qi Rong's and An Le's plan to kill everyone from the Young'an royal family including Qianqiu. Xie Lian finds the king injured but not dead, while trying to keep him from dying the king not only demands that An Le must die but that everyone from Xianle must die as well so in order to keep the peace Xie Lian kills the king and An Le. Qianqiu chases after Qi Rong since he's the mastermind and demands that the matter between Xie Lian and him be settled between the two of them and for him to receive no punishment.
@Amaya I know it hasn't been explained properly yet but Young'an was actually apart of Xianle originally. echnically Young'an are Xianle xD but I guess you could say that as much as North and South Korea use to be one.