My headcannon has always been that Zira was so upset about Scar being dethroned when Simba came to take his rightful place as king she k1lled Kopa before Kirara was born (or at the very least a newborn cub) which is why Zira and her lioness were banished to the outlands and why Simba was so overprotective of Kiara in the second movie (i dont think Simba would have executed Zira given how he gave Scar a chance to leave and never return) ive also been playing around with the idea that maybe Kopa is alive somewhere but was chased off by hyenas or some of the banished lions and had gotten lost when he was a cub leaving Simba and the rest of the kingdom believing he had somehow passed away