What if Kaido is preparing Luffy, Yamato and Momonosuke for the big war?
I mean, Kaido realized that things could change after seeing Luffy and Yamato's strong determination.
Also, note that Kaido is baiting Yamato about her Oni lineage. So he is waiting for the moment "so, what you gonna do, Yamato?".
The man dies making a glorious purpose and an smile on his face. Whitebeard, I've reached you finally!