I don't think Simba was all that bad, honestly.
Sure, he might have been super excited about the fact that he was going to be the next king, and he might have allowed it to get to his head a little, but come on - he is just a cub. How would your 10 y/o self act if they found out they were the future heir to a kingdom. I know I would have been as obnoxious as
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Most people point out his behavior towards Scar and Zazu, but I don't believe they are putting themselves in Simba's shoes. He probably expected his uncle to be happy for him, and he obviously likes Scar a lot judging by their interaction. As for Zazu, well, it looks like Zazu is in charge of teaching him some things about his behavior and looking out for him, and that annoys Simba because he is standing in his way of having fun. If you had that one boring nanny that always kept you away from mischief, would you like that? Besides, Simba obviously cares about Zazu enough not to leave him behind, and there is some negativity for Simba's antics from Zazu's side too. All in all, I don't think Simba's behavior is in any way overdone with childish selfishness, especially if we take in Nala, Sarabi, Timon, Pumba, Sarafina... all who Simba seems to respect.
One thing that I've noticed that people rage quite a bit about Simba being a selfish brat, but when it comes to Kopa, who generally shows even worse behavior, people are more forgiving. Why?