If ever Saya succeeds in her assassination attempt, she will be removed by the Human Army immediately and executed. Then, the Emperor/Mu Xuanjue would be in the situation where he could get overthrown by the Army, as was the case with General Li Yuan who toppled the Sui Dynasty and became Emperor Gaozu of Tang. The main reason the Emperor seems to be tolerating the Empress Dowager is the fact that he might fear the Army disposing of him if ever she were to be removed. He and the Empress Dowager live in a sort of equilibrium. And she might fear the social backlash that an Empress Regnant would stir up if ever she were to completely remove the Emperor.
The goal of Saya's assassination attempt was not to seize power. It was not even to assassinate the Empress Dowager. It was to go out with a bang that would reach the Mirage Country and send a message to her people who might very well, in the future, face the Empress Dowager's attempts to annihilate all foreigners. Whether Saya is there or not, they will have to fight. Not even farm animals die without putting up a fight of some sort.
"Murderous bitch" would not exactly be the theme I would have chosen for this chapter, however.