[MUST READ] How did Itachi..?
How was Itachi able to defeat Nagato? It shouldn't have happened. Crazy, right? No. Here's why:
Itachi was able to throw kunai knives into both the King of Hell's eyes. But how could he do it?
The King of Hell can only be seen by the ones it's being used upon and The King of Hell only appears when someone using the Naraka Path grabs you physically.
Naruto and Bee were the only one's in Nagato's grasp.
The King of Hell doesn't take on a solid form, so how didn't the kunai just phase right through it's eyes?
It isn't shown here well, but in the anime, when the Naraka Path kicks Ebisu, he flies right through the King of Hell without him even realizing it is there.
So essentially, plot was the main reason Itachi was able to overcome Nagato.
Had the kunai never hit, Itachi wouldn't have been able to save Naruto/Bee due to the shared Rinnegan vision, and they would have lost.