Yeah, this is something I always wondered. According to the movie, Zira trained Kovu to kill Simba, but it never really tells us how she trained him. Did she make him train with the other lions, or did she just make him hunt, or feed him well? XD
They never really said how Kovu would kill Simba either, and personally, I feel that Kovu wouldn't really win against Simba in a fight, because of the difference in age and size >.>
So what do you think? :B