the problem is that Kiara and kovu didnt take long enough to settle their issues. Kovu has just been exiled, something that is supposed to last for years, at least. hes been rejected by his family, meaning hes supposed to spend at least some time- weeks, but more likely months- wandering around the savannah looking for a new home. and Kiara doesnt get to run off in open defiance of her father and came back home without penalty, i e Simba should have stripped her of her birthright for her actions- a role that she didnt even want as a kid. " you have not shown to be reliable and trustworthy. I must look elsewhere, for this prides next leader." as for Kovu, returning to the pride would have been a certain death sentence, since an exile is the harshest punishment a king can give out short of execution, and exile is always 'upon pain of death' ie you come back, you'll be killed on the spot ' Kovu should have been adamant about not coming back for that reason, because his neck is on the line, not Kiara's, although truthfully a pissed off Simba may well disown her and order her execution as well for helping Kovu.
this isnt a decision you make lightly, as the price you pay for making it, is extremely high. theres no walking it back. Simba didnt show Scar any mercy, because Scar would not show him any, it was a kill or be killed situation. mercy is a luxury you have after you win, then you can be as benevolent as you want to your beaten enemies. Teenagers dont make good leaders, because they dont have the experience and wisdom needed to guide companies or countries successfully. kiara was the weak link of the pride, gullible and naive and something Zira should have exploited more, or targeted more. If you cant kill a king outright, you cripple his legacy, by taking out his kids. Kiara , being in the pride the entire time, should have been aware of what was going on re Zira and the outsiders. to be ignorant of such things when you should know about them is not good leadership. Simba because of his long absence, could be forgiven for not knowing anything about - or not caring about - what was going on back home. kiara doesnt have that excuse.