I thought I'd give some background info on the Prime Minister, her role, how she got where she is, that sort of thing. I'm just not sure how.
Still, there's something I don't understand in your questioning (which others have done too). For me, it feels it's like:
"Why doesn't the knight who lives her life with the idea of honor, who cares very much about her oath to the country she's sworn to protect, doesn't simply kill the prime minister, the woman who's been elected as a leader?"
I don't know, I feel like the answer is pretty obvious, no?
To put it another way, Theresa is an interesting character for me because she confronts her ideal with the harsh reality. She chooses to be a knight, convinced that she can do good by serving a just and righteous leader. And then she finds herself with the Prime Minister at the head of the country, someone who is far from what she wanted. And yet, she is the Prime Minister her country has chosen.
I suppose people would want her to leave, but that would mean leaving Angelica alone, and it would mean giving up doing what she's wanted to do all her life, protecting the people of her country from the various threats they face.
There's more to say about her, but that's for other chapters. I'm not sure it helps much, but... that's how the dynamic between the PM and Theresa works for me. A good person who is forced to serve a bad person. And is still trying to do some good. That kind of idea.