In my opinion, Kovu was brainwashed from when he was a kid, and therefore was not his fault he was so evil at the beginning. He was very small, and anyone would become evil-like if their mothers are training them to do so, from a very young age, like in Kovu's case. He only realized he was doing very wrong when he fell in love with Kiara, and so betrayed the Outlanders, including his mother.
The reason, I believe, that Kiara stole away the "evil" from him so fast, is because he was never really evil at heart. That's what I believe, because if he had really been evil at heart, he would have proceeded in killing Simba, right?
And also, IMO, Kovu is an awesome character!
Jus' my two cents on the subject. ^^
