Perhaps in the fanfic's plot, Naruto discovers some dark secrets about his origin or the ninja world that make him feel so disgusted or hopeless that he wants to die. Like if he finds out that the very system he has been fighting for is corrupt at its core.
Luffy beats Kaido by unlocking new powers and exploiting Kaido's weaknesses. He also gains support from his allies and uses teamwork to his advantage. The battle is intense and filled with unexpected twists.
The chapter where Luffy defeats Kaido is 1049. This battle showcased Luffy's growth and determination. It also had a ripple effect on the other characters and plot developments in the series.