Zira doesn't look like she has many friends in the Pridelands. It appears that nobody ever questioned Simba throwing her out. It's possible that, despite being a bad leader to most of the Pride, he could have been good to her. Simba was gone mysteriously for years then turns up and then outright confesses to being responsible for Simba's death. Before Scar can execute him, Simba jumps up and attacks Scar and makes him confess to killing Mufasa. Now, we know that Scar is guilty, but if Zira were present and were watching the scene, she may have thought Simba forced a confession to frame Scar. Then, after the fight on the top of Pride Rock, if she hadn't known that Scar had survived the fall and not fallen into the fire, she wouldn't have known that it was the hyenas who actually killed him.
Then, perhaps just for not feeling Simba was that rightful king, Zira and her supporters and even her kids were kicked out and sentenced to the barren Outlands where there was little food.
Given how strongly Zira feels that she was wronged in "That's My Lullaby" and the only mention outright of killing someone was Simba (what exactly she planned to do to Kiara is unknown as all she said was "squealing in my grasp").
It would also explain why, in Lion Guard, she didn't kill Kion on the spot, if she her main target was Simba. (Perhaps "squealing in my grasp" just meant that she would take the heir to the throne, Kiara, and hold her prisoner, thus forcing the Pridelands to surrender after Simba's death.)
Also, if I recall from Simba's Pride, she said that the fire rescue was planned, meaning that, unlike some think, she didn't intend for Kiara to die in it. Nuka just got out of hand and she got into more trouble than was planned.
Then, right when she thinks Kovu has Simba, she realizes that Kovu has turned to the other side. After Nuka dies trying to kill Simba, she feels guilty for having neglected him all of those years and instead focused on Kovu, who had just betrayed her.
Then Zira tries again to get at Simba, but is first thwarted by Kovu, then Vitani turns on her, at which she really starts to lose it and says that if Vitani won't join in, that she will die, at which point her whole pride betrays her.
In desperation, she tries to kill Simba herself, but is thwarted by Kiara. Rather than surrendering to those that she feels are responsible for the death of Scar and now, perhaps in her crazed mind, Nuka, she lets herself fall to her death.
Unlike Kovu, I'm not sure she ever learned the truth about Scar: that he really did kill Mufasa and that his death was caused by the hyenas, not Simba.
More to the point, it's quite likely that Zira herself was just a pawn to Scar, yet she never was able to realize this.
If all of this is true, she's got to be one of Disney's most tragic characters.