In Chapter 999 Big Mom tells Kaido he is free to kill all of the Straw Hat Pirates except for Nico Robin. Kaido asks Big Mom if Charlotte Pudding can read the Poneglyphs, but Big Mom says no and she doesn’t want to wait anymore.
Then Big Mom asks if the Road Poneglyph is in the Flower Captial, but Kaido tells her that she is playing her hand too quickly by asking that.
So Kaido is fully aware Big Mom is using him. It’s consistent with Kaido’s words to the Red Scabbards.
Big Mom yells for him not to insult her because even after everything, she considers him a little brother to herself. She then reveals that on the day Rocks fell, she was the one who gave Kaido his Devil Fruit: the mythical Uo Uo no Mi. Because of that, she believes he owes her a debt for life.
But Kaido knows betrayal is right around the corner. Loyalty and trust have been a constant motif of the Wano arc. It starts with Law’s dubious activities jeopardizing his alliance with Luffy, the total collapse of Kid’s alliance, Apoo’s disownment from the Beasts Pirates, and a Samurai’s oath to their lord. And now we have the main antagonist harboring a morbid perspective on pirates and alliances.
They definitely stem from Rocks D. Xebec and his gathering of many powerful figures and it not working out.
When you have that many extraordinary figures on a crew, coercion and submission will not work. This is exemplified perfectly with the Straw Hats, a crew composed of exceptional members connected by an unbreakable bond forged through trust.
*Theory by KiriNigiri