I know everyone has their theories of what Kion could have been, and here is mine. I will be posting information/images of the other major characters in the story. Heck if I'm inspired enough I may even draw a few scenes.
When I think of this "evil" Kion, I imagine a lion who started off very happy with his position in the pride and who was probably okay with not being King. In this headcanon Kiara displays a similar personality to TLG: Extremely spoiled and rubbing it in everyone's face that she's going to be Queen.
Anyhow, Kion here is a great leader of the lion guard, and he thwarts many dangers to the pridelands, including Zira. He doesn't get as much credit as he deserves and eventually he begins to think that he would make a better ruler simply because he's proven himself capable so many times while his sister has not. Eventually this starts to get to him and he decides to take things into his own paws. At the passing of his father, he banishes his sister and kovu as well as all their followers, then institues a fierce system of loyalty and punishment. The Pride Lands don't whither per se, but fear is readily rampant in all the animals and Kion is extremely harsh in his dealings. The only member of his pride that he tries to be gentle with is his mother, who is so shocked and torn up about the whole thing that many think she's gone mad.
Sorry if this didn't make sense XD It did in my head.
Kion and images (c) Disney