It's obvious to me that the show is trying to make Su Fu a kind of tragic character or tortured soul to elicit sympathy because his trajectory means that ultimately he can't be with the female lead. The son of a concubine... so of lower status, somewhat despised perhaps and then trains as an assassin and then embroils himself with rebellious elements working against the throne. With a CV like that I can't imagine that any man in his right mind would think that he would make a good husband for a woman unless he's using her as his cover story. But that's probably it... he isn't in his right mind now that the drugs are taking firm control over him. I suppose he thought he was doing his bit to protect RY when he became a willing puppet for the chief of the secret organization but who's going to protect him from himself? I can't help feeling that the more the show is trying to sell him as some kind of viable alternative to XS because he really loves RY, the less I'm inclined to buy into that idea. I don't think that there are any alternatives. XS is the man for her. SF is a distraction but more importantly he presents imminent danger to everything she holds dear.
The rapidity with which she friendzones him begs the question as to whether she was really in love with him in the first place. I have my doubts of course. I tend to believe that it was pity that brought them together. After all she still pities him on some level. She mistook that for love and realised it in time. I don't think it's because he deceived her per se although he did. Or used her. Which he also did. It's more the fact that she realised she had entered into an illusory romance when all the signs were there that things were not as simple as she had wished. Their common background or past experiences did not necessarily mean a common future or common goals. RY tends to be somewhat self-righteous and not a little arrogant relying on her own ability to make correct judgments when in reality there are bigger things at play that she's not made aware of.
There are remnants of the previous Sui dynasty who are discontent with the current regime... although why that is, isn't entirely clear to me at this point in time. The general populace don't seem to be doing too badly as a whole to warrant a change of regime. It could be that well they don't care much for the Li family and they're purely Sui loyalists. So it behooves them to start a war... a coup de tat so that the old regime be reinstated with no thought to causing death and destruction to the masses.
RY's journey to finding true love parallels the cases that she and XS have taken on together. The shadow puppet display she performed with Wan Lu to jog his memory confirms that. It's a tacit acknowledgment of their special relationship and how she can't achieve the detective gig without him. He seemed moved by the gesture but still it's not enough for him to be forthcoming and so he maintains his distance because ultimately he wants her love and devotion not a continuation of their quid pro quo relationship. Also, he's well aware that his enemies are only too eager to use her as a means to get to him and his mission so his decision to remain aloof to her entreaties have a twofold purpose.
As for the beating she took for meeting up with the bag man, that was undoubtedly painful but probably not as painful for her as being ignored and misunderstood by XS which is for me a far more satisfying state of affairs between them because now the shoe is in the other foot. It's eminently fascinating that the cases that they've undertaken together have paralleled to some degree her journey to "finding true love". All of the cases have been analogous or have telegraphed their somewhat tumultuous relationship. They are morality tales of missed opportunities, obsession and lost love. In the case of the erotomaniac serial killer, the show attempts to draw a comparison between the situations faced by the couples. At least RY herself is aware of the comparison when she chastises the hostage taker for his foolish pinings. There is bitterness and despair in her words. She had hoped that XS' love was big enough to cover her sins but alas it was an illusory promise. Before the man she was really in love with, it was an indirect confession tinged with resentment and regret. It is tacit acknowledgment that now she has indeed found true love only to have lost it. All she can do now is live with the pain.
At first XS thinks she is referring to SF as seen in the stern glare he shoots the other man with while she monologues. But when he goes home and reflects on her words, he wonders if he's misunderstood her relationship with SF and by extension himself. From his conversation with his surprisingly progressive father, it's clear that he still cares a great deal but he can't repeatedly allow himself to be in the position of being betrayed because he thinks that she is in love with another man. He has always respected her freedom to choose and he also has to be careful because of the larger political matters that are at stake especially if SF is persistently using her as a conduit to him. He can't be sure of her if she's trying to keep both sides happy. What's really interesting to me is that for her, she thinks she's lost his love whereas for him he believes that he's never had hers in the first place (largely because of all the contradictory signals that she sends out).
I can't judge her too harshly because of her inexperience in such matters and her world was small until she invaded his world unceremoniously. But she is learning a really important lesson here. She can't take XS for granted even if he is ultimately true to her despite the appearance of rejection. Even if she doesn't want to, she has to draw the line and be clear about who it is she preferences. Everyone wants certainty, XS does too. He needs it for all kinds of reasons not just emotional ones. It's not easy for him because he is also a servant of the state as much as he is the man who loves her.