At Whole Cake Island, Big Mom contacts Kaido via Den Den Mushi, telling him to stay out of her way as she goes after Luffy, saying that she will be humiliated if she isn’t the one to kill him.
Kaido refuses, citing his own grudge against Luffy and threatening to kill Big Mom if she interferes. Linlin then says that he owes her a great debt from a long time ago and insists that they cooperate on this like the “good old days”.
So what is Kaido’s debt to Big Mom? To be honest I think it has a connection with Kaido being unable to die. We are all to believe that him being a dragon is to blame (Like imagine putting a little seastone cuff on a massive Dragon with a humanoid devil fruit: Poof! Gone human form and BAM dragon! No cuff will hold you, etc…).
Still, dragons are not unbeatable and I think that Kaido was almost defeated in the past as a human and as a dragon (just as what he is). Big Mom saved Kaido like any crewmate would do but Linlin did it without thinking it really through.
Kaido has 2 problems as we know:
-He’s unable to die by others or by suicide;
-SMILEs production trampled to nothing thanks to Law and Luffy;
For Kaido to be soooo damn ungrateful to Big Mom it has to be his inability to die. Kaido suffers from boredom and being immortal. We have one connection to being immortal and that is Law. It is not weird to think that the previous owner of Law’s Devil Fruit used the ultimate move on Kaido to make him immortal. But that would maybe be too easy and Big Mom’s power is extremely OP. I don’t want to make a list so here is one example and let the creative mind of your own do the rest:
-Linlin took out the mortal part of Kaido’s soul and put it into something else.
Since Big Mom is in Wano right now and Kaido is well known for being unbeatable, maybe Kaido will be defeated after Big Mom undoes what she did to Kaido all those years ago.
*Theory by ChuckTalens