Nala's main opponent was Vitani.
So if Nala was killed during the war which was originally started by Zira, Simba would have ran over to her screaming her name. Then Simba would probably try to kill the Outlanders.
But Kovu and Kiara would by interrupt
them. With her "We are One" crap.
If he saw her die, "NAAALAAAAA!" or "NOOOOOOO!" and he'd obviously run over to her body. If he didn't see her die and someone told him, probably a similar way to how he reacted to his father dying.
He would be devastated, depressed, and lost, because Nala is his best friend and love companion, if Simba was in a mood he always has Nala to cheer him up. If Nala died, he'll almost go suicidal (the only thing that will keep him go suicidal is Kiara).
