Theory

I have a feeling that Zin’s mother was part of royalty. Let me cook and be delusional for a second. Zigrien’s dad has an illegitimate child with a woman of the royal family. And since Laurent wanted to eliminate everybody in the family that could pose a threat to him, he found out about this and sent people to kidnap and kill Zigrien. Idris, to save him, probably had to kill Zigrien’s mother.
And this is what Laurent means with “would wh respect you if he knew what you did?” And Laurent has allowed Zigrien to live because Idris followed with and killed Zigrien’s mom and then everybody pretended that Zigrien was just the bastard son of a marques. Since the mother was dead, there could be no way to prove that Zigrien also has royal blood. So, that’s why Idris keeps protecting Zin, because if the truth comes out, then Laurent would not hesitate in killing Zin, who by having royal blood, could pose a threat to the throne for him.