We see Zira as the 'bad guy' who threatened Simba's Pride, but what if she DID try to forgive her foes and accept him as King? It could be possible that Scar's followers were treated as 'outsiders' and maybe even Simba became paranoid whenever he looked at them. Since they do have an uncanny resemblance to his uncle Scar.
Kopa (Though he may be interesting) isn't canon.. but say if he WAS canon.. what if 'Mheetu' and Chumvi' were corrupted by Scar and they attacked Zira and her Pride when Simba and his Pride were away. Though it's always about Zira being vengeful and murdering Simba's son. But WHAT if Zira was protecting Kopa from the bad lions and he fled the Pridelands.. but Zira, covered in blood after the horrific battle. Slept in the Royal Den and as she awoke. Simba and Nala return but are angry when they don't see Kopa and Zira's blood-covered paws. They think SHE killed Kopa and exile her.. She snapped and went into the 'insane Zira' we know and love. But is it too far fetched? I think that parents go 'insane' when their child is in harm's way.. it'd wouldn't be too 'Out of Character' for Simba or even Nala to blame Zira but not knowing the truth. In fact, I think that's what would of turned him into the 'over protective father' in SP. Nala however, may have knew of Zira and what she was capable of. It's just a theory.. but what do you think of it?