Sorry Zira..

..."I'm afraid I can't actually help you, I mean; I can...but I won't. You see, If I were to let you live, you'll likely just try and kill me and my family again. Your insanity and unwillingness to change is too hard to ignore. If I'm to be Queen someday, I need to make logical decisions, and it wouldn't be wise of me to let a murderous psychopath loose in the Kingdom! Daddy is coming down so I need to make this fast, it won't hurt or anything. Many of our ancestors have died in a similar way, it's always pretty quick. Then me and your son are going to celebrate our two and a half day anniversary by getting married! And I sure as hell am NOT having you as a mother in law! Don't worry, Kovu will be fine; he'll probably just give a disappointed sigh and then forget all about you immediately. Goodbye Zira." *throws Zira off the cliff*
"Kiara."
"Oh Daddy...I tried."

--------------------------------

Heh, what if Zira falling wasn't an accident? And what we saw; was Kiara's 'recollection' of what happened that day? No one else saw it after all. I know, that wouldn't have happened. They were trying to make Kiara look noble by trying to help a psychopathic killing machine, but what did she expect was going to happen if she succeeded? Zira said herself "I'll NEVER let it go!" Zira instigated the war herself due to her own delusions, brainwashed her children, she tried to kill Kiara's father twice (three times if you count The Lion Guard's depiction of why the Outlanders were exiled), threatened to kill her own daughter for turning against her, violently refused help when offered, and yet Kiara was still willing to let her near her family again by saving her! Noble? Perhaps. To take pity on someone who has clearly lost their mind is one thing, but there was no help for Zira. She was too far gone, and there was no way to convince her otherwise. She was hell bent on cold blooded murder of the current royal family, given the chance; she would find a way to succeed with or without her followers. Seems disrespectful to Simba to save the one lioness who wants him dead more than anyone! Seems virtually everything Kiara does in this movie is a middle finger to her own father.
It's like if you saved Ted Bundy from certain death, yeah you saved a life, but he'll likely then turn around and murder you; and go on to maim multiple other people to death, so...worth it? Shouldn't the 'greater good' dominate the decision here? I guess it brings up a moral dilemma and challenges a very basic human instinct, life preservation. Pertaining to a pretty challenging philosophical question, would you directly take a life, to potentially save multiple lives? For example, 'if you could travel back in time, would you kill Hitler while he was a baby?' Not only does this raise the question as to whether or not you could physically kill another person (and a baby no less), but also considering that if you did choose to kill him, could the consequence of doing so; end up worse than what was originally destined? Taking philosophy in High School used to keep me up at night with questions like this; let me tell you!

At least when Simba did something similar in the first film, Scar was family; and thus his decision to spare his life made a little more sense from an ethical standpoint. And Zira's followers were disgusted/disgruntled rather than betrayed, so them turning on her; might not have been absolute if Zira were to survive long enough to manipulate them again. And then the cycle could begin again! In general, saving Zira made no sense logically. Luckily gravity took care of it, though I wish Disney kept her suicide scene. 

The Lion King 2 Simba's Pride (c) Disney Toon Studios