I just got whole idea behind Kaido's words:
"It's been a while since I've been this fired up... but I've made a mistake, I should have cut off your head and declared 'victory'.
Because people won't stop believing in you, right?"
Kaido looks super angry/frustrated and also disappointed. He says that he made a mistake, because he should have killed luffy, as you know Kaido could easily transform into the dragon form and kill luffy flying down the sea. However Kaido does nothing, why?
Because the words "Because people won't stop believing in you, right?" are meant to Kaido himself. He was fired up in the battle because he finally found a guy who possibly could kill him.
Kaido wants to be killed in the fair battle, facing an overwhelming power and that's why he can't kill luffy right now, because he still believes in his victory, that's why Kaido's face is so tragic. ( Kaido is luffy's fan right now. Kaido: "Come on Luffy, you could kill me, just do it" )
What do you think?