Also, Kovu's death would be a setback for continuing the monarchy unless Kiara remarries.  (Unless she is dumped as the heir and this new Kion takes her place.)  In short, it woud undo a part of what movie 2 was all about.

(It would be as pointless as if they were to bring Bron in in Great Longneck Migration yet kill him off at the end of Journey of the Brave, bringing this huge plot change in in 10 for nothing.)

As for Kovu, I'd hardly call him a villain or anything near it in movie 2.  If anything, he's an antihero.  Indeed, even Zira is more sympathetic than Scar.

Let's put it this way, Zira was likely fond of Scar over Mufasa (perhaps she didn't like Mufasa, as it would be niave to assume every lioness in the Pride Lands was a Mufasa fan.)

Anyway, when Simba first returns, reappearing after an absence that is hard to explain (he was supposed to be dead) and why did he run away?   (We the audience know, but nobody else but Scar and the hyenas did.)

Simba actually confesses to having a role in Mufasa's death right in front of everyone, and, when his own mother asks him if it's true, begging him to tell her it's not, he outright says it's true.

Then it looks like Scar is trying to finish him (which he is).  None of the other lions come to Simba's defense, just letting Scar corner the rightful heir on the edge of that edge.  It seems they believed that Simba deserved it.

However, Scar ran off his mouth, letting Simba know the truth.  Then Simba knocks over Scar and makes him admit his guilt, which he does.  Then, the lionesses aid him.  (They weren't too fond of Scar anyway.)

Later, Scar is kicked by Simba, off the cliff, in sef-defense of course, but Scar survives the fall, but is kiled by his own henchmen.  

However, it is unlikely anyone but the hyenas knew that Scar didn't die by Simba's hand.

Thus, whie those who hated Scar would believe Scar's confession over Simba's, those who were unsure or were pro-Scar would think that Simba caused his father's death, ran away, came back and tried to depose Scar, confessed to killing Mufasa, attacked Scar and forced a false confession out of him in his brother's death, and then murdered Scar so that he could take the throne.

And, of course, Simba kicked them out.  So Zira may truly have believed Simba to be guilty of both Mufasa's and Scar's deaths and that he wasn't worthy to be king of Pride Rock.

Also, Scar had picked Kovu was his heir.  If anything, Zira's main flaw is how devoted she is to Scar.  In fact, her last words "This is for you Scar!" confirm it.  

Indeed, she tries to raise the appointed heir, Kovu, and neglects Nunca, and it's clear that Nunca is so upset that he's almost ready to have history repeat itself (i.e. him kill his brother to seize power just like Scar did with Mufasa.)

However, Kovu, despite hearing how bad Simba has been his whole life, actually decides to hear Simba out on the Scar thing, rather than killing Simba when alone.  (Perhaps he DID plan to kill Simba if Simba had admitted true guilt (though, admittedly, that's not a bad trait), prompting Zira to act herself.

Nuncas ends up dying, and, ironically, his death turns her against Kovu and pro-Nunca, giving him more attention in death than she had his entire life.  

She attacks Kovu, giving him a scar, and blames him for Nunca's death.  Indeed, she practically disowns him.

Kovu actually goes back to Simba, even though he's got to know that Simba could well have him executed on the spot for, as it would appear to Simba, attempted murder of the king of Pride Rock.  

After he leaves and Kiara comes to him, I think he believes that neither Simba nor Zira are going to back down and so the best bet for them is to run off and form a priide of their own.   It's a wrong assessment, but Kovu isn't perfect.

Anyway, we can see how fanatical Zira is when she threatens to kill Vitani merely for not fighting, cuasing her whole group to turn on her.

Kovu was pretty good to get past Zira's brainwashing.  Not everyone could do that.  So don't call him a poor leader.

You know, the type that serve some weird deity or spirtula force and have funny rituals.

Perhaps some could try and sacrifice the Gang to volcanoies to please some deity.  The fun of this is that they could be a bizarre form of Leaf Eater, Fast Runner, Hidden Runner, or Egg Stealer and need not be Sharpteeth, but still could be interesting bad guys.

Or they could at least be an amusing distraction to the plot.

Ruby learned that she would be in Journey of the Brave.