Nala's main opponent was Vitani. So if Nala was killed during the war which was originally started door 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 door interrupt them. With her "We are One" crap.
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).