In chapter 1049 we see some pirates reacting to the news of the Rocks pirates being beaten and of them says “I bet their infighting did them in. None of them were team players…” The next panel we see is of a young Big Mom who looks pretty upset and says “Don’t screw with me! Where the hell did you go, Kaido?” Then the next panel is the Kurozumi granny saying “10 years have passed since that incident, Kaido… You’re now known as the ’embodiment of brutality’.”
The first two especially implies to me that for some reason during the fight Kaido decided to stop helping Rocks. The third one implies to me that something happened at God Valley which contributed to Kaido becoming the “embodiment of brutality” and the way she says “that incident” instead of just saying what happened could mean a couple things as well.
Big Mom gave Kaido his devil fruit as he was on the brink of death. The way I see it, Kaido got his shit rocked at God Valley, was given his devil fruit by Big Mom and once he learned how it worked, turned coat and fled the scene.
Maybe Kaido was ashamed that he suffered such a defeat. He was ashamed of brutally abandoning his former crewmates to save himself, a side of him that still seems to come out when he’s drunk. That’s why Big Mom is angry that she cant find Kaido in that flashback.
With an insanely strong devil fruit along with his already impressive strength. It’s not surprising that Kaido would become the embodiment of brutality. It seems like Kaido holds a lot of resentment towards himself and the world. I think this is because something happened at God Valley that showed him the true colours of Rocks.
That could be the reason why Kaido is so intent on breaking the spirit of the hot rookies and so sure that Luffy’s alliance will end in a betrayal. He wants to believe, that under enough pressure everyone would have done what he did.
Theory by Veidovis (https://www.reddit.com/user/Veidovis)
