I was thinking about this, and while, obviously, she would know that he took over when Mufasa died, did she know Scar was responsible for that? Maybe I'm remembering incorrectly, but I don't recall Zira ever saying anything at all about what Scar did as king, or how he became king. Now, this could be because she didn't want to let her kids, and especially Kovu, know about the true nature of the lion they were fighting to avenge. It would probably be problematic if they knew the truth of the story. However, I was thinking about a different possibility.
What if Zira legitimately did not know that Scar was responsible for Mufasa's death or Simba's flight? If she simply believed Scar's story that Mufasa died trying to save Simba, her vendetta against Simba makes a lot more sense. If she truly believed that Scar was the rightful king and had taken the throne legitimately in the aftermath of tragedy, it is completely understandable why she would hate Simba so; if this version of events were true, then it would be Simba who killed (indirectly) the legitimate king and deposed him. Not to mention, Simba fleeing in the aftermath of the death and then returning later would make it seem likely that he was in some way responsible from an in-world perspective. And it would explain why she was so defiant and zealous even after the rest of her group switched sides; she would legitimately believe her cause was just and that the other side was evil, choosing to die instead of joining them makes sense in that case.
What do you think, is Zira simply being unaware of the full story behind Scar's rise to power a plausible explanation for her behavior and devotion to Scar even after his death?