He tried. But those he saved have been taken hostage by Tuan.
To make matters even more complicated Rong-lu has his cadre of loyal troops, and he has no intention of handing his prisoners over. Tuan may think that he still has forces to command and a war to lead - but Nieh and Tung are basically conducting their campaign ignoring Tuan, and no one can thus enforce court commands Rong-lu doesn't want to comply with. Such as handing the prisoners over.
Hostages are one thing. A massacre is another. If Rong-lu, acting as an Imperial general, protects some substantial number of the Western civilians in Peking, and then delivers them safely to the relief force, he re-establishes the good faith of the Imperial government.
He realizes quite well that this is his best shot to survive this disaster.
That hasn't happened. All whites in Peking have been murdered or are hostages (and the numbers of each are not clear). This was by action of the Imperial government, which has done
nothing to offset these crimes.
And has in fact declared war against the Powers, who in OTL chose to ignore this. This time they don't really have this option - but right now no one wants to deal with the logical consequences of dismantling the Empire, since dividing the spoils is prone to bring even more discord among the Alliance.
is over. Who is ever going to trust the Imperial government again? They are treacherous murderers - all of them, as far as any Westerner can see. No deals. Rescue the hostages, and destroy the criminals.
The powers have already demanded their heads on a plate - aside from the hapless Emperor himself, of course. The court reactionaries have therefore dug themselves into a deep pit. Now the diplomats at Peking are trying to figure out a solution that would satisfy the need of a suitably harsh vengeage, while still creating a postwar situation that would the Powers to exploit China without having to directly occupy and administer the whole area. Since the Boxer War has already proven how impractical and outright impossible such a solution would be, finding someone to serve as proxy for their policies is pivotally important.
Meanwhile the Southern viceroys are trying to play the foreign barbarians against one another, and the literati reformer-revolutionaries are trying to keep their more radical comrades from agitating the commoners too much. No one is happy with the way things are headed, but finding a way out seems increasingly difficult.