I need some help understanding a part

Personally, I think the king wanted leverage from Bo. The king knows that lord Bo has the capacity to over throw him. Most likely fear drove him to make Xing'er a pawn. If you look back, the king has a tendency to keep everyone on a short leash.

The narrative is kinda inconsistent throughout the story. However, the most reasonable explanation is.

The king is aware of their relationship but he wasn't aware of the depths of their relationship VS the depth of Lord Bo's loyalty to him. He married Lord Bo with MZX because he believed his loyalty to the king is strong enough to overcome his love for her.

As for hiding the fact that he had the night furry massacre the Ma family behind the MC's back. The story answer is that he thought Lord Bo was to soft hearted to go through with the idea but the king still wanted to severe Lord Bo from his wolfie persona to create the ultimate warrior with no weakness.

If you asked me however. The kill order behind the MC's back is to give him wiggle room to make a moralistic argument later on that it wasn't technically him who killed her family so he's technically not at fault for their death.

We can both agree, however, that Lord Bo totally would've killed her family if he received the kill order.

 Julie:

I thought the King was aware of Ma Zhai Xing and Prince Bo's previous relationship? I thought that is why he had the Night Fiends murder the family behind Prince Bo's back. Did I misunderstand that part? 

My understanding of the situation:

The King's knowledge about Bo Wang and Ma Zhaixing's relationship was limited to his exposure to them during his trip to Kuizhou City 8 years ago. There, he saw Ma Zhaixing tried to clear Wolf Boy of the crime of assassination of Xiahou Yi, and how desperate she was to find Wolf Boy after he fell down the cliff injured. From there, he saw how sharp and clever Ma Zhaixing was, and her compassionate heart. I don't think he realised the the two of them were in love because 1) they were both so young then, and 2) he probably thought Ma Zhaixing was simply being a righteous girl who wanted to clear the innocent, and a girl who felt really upset over the fact that Wolf Boy was hunted and killed because he was accused of a crime he did not commit in the Ma Manor so her family was partly responsible for an innocent young boy's persecution and death.

My read of the King secretly having the Night Fury team murder the Ma Family behind Bo Wang's back was because the King knew that Kuizhou City and its proximity to Langshou Mountain where Bo Wang was brought up might have made him feel sentimental. I get the impression that since Bo Wang was saved by the King at the foot of the cliff, he had never been back to Kuizhou City so the King wasn't sure how Bo Wang would feel being confronted with a past which he had deep and strong memories in. 

And like most people here, I also totally believe that had Bo Wang received the kill instruction, he would have carried it out without hesitation or misgivings because he had no ties to the Ma Family. He had no idea Ma Zhaixing was a daughter of this house prior to the Ma Family Massacre. 

Chinese censorship board and its insistence that a main protagonist of a show needs to be white, not grey or black, is probably what altered this script because from the actual drama footage of the Ma Family Massacre, I think they really filmed with with Bo Wang among them doing the killing. I mean, those central shots of this massive hooded figure with that pronged spear marching through Ma Manor must have been Bo Wang. That was his body build, his hooded outfit, his boots and the blade end of his spear looked absolutely similar to the short knife he later gave Yao Ji. I can't help but think a lot of the 'problematic' parts of the show had to be modified or cut to comply to the Chinese drama requirements, which was a great pity...