I don't know what Zira did, but if it was something as petty as disagreeing if Scar was good or bad, Simba seems as bad as his uncle in sending the young cubs Kovu, Vitani, and Nuka to the Outlands like that. What did they do against him?
Again, Simba kicking them into the Outlands gave them a PERSONAL reason to hate him, just like Scar sending Simba into the Outlands for years increased his hatred for Scar.
Also, more to the point, he seems merciful enough to have let his uncle who had
1.) Admitted to murdering his father
2.) Just pimp slapped his mother.
3.) Mismanaged the Pridelands and been about to have them be stuck there with no food due to his own stubbornness.
4.) Had just tried to murder him as a cub and then recently as an adult
5.) Had lied to him and made him feel guilty for years for his own father's death and had told him to leave the Pridelands where he nearly died in the desert.
6.) Had, in the musical and the cut movie, tried to make Nala his queen by force
leave the Pridelands alive, then why didn't he cut Zira's cubs any slack?