I think that people shouldn't be surprised that Kaido is upset that Luffy lost.
From the very beginning, Kaido wasn't contemplating he was gonna kill luffy.
He got pissed at Luffy for ruining his smile business by taking down ceaser and doffy, but when he first defeated him, kaido didn't immediately kill luffy, but just locked him up in prison.
When the raid came, kaido allowed luffy to attack him first without resistance, to which luffy succeeded as this was the first time he attacked with Advanced Armament Haki.
After Kaido beats supernovas in Round 1, he comments how its a shame that they didn't join his crew, as kaido believed that with their strength together they could have conquered the world.
He let kidd and the others retreat and stayed to fight luffy alone.
And now after defeating him AGAIN and with luffy falling into the sea kaido says he's disappointed.
I think kaido was thinking of 2 things:
1. That luffy wasn't the strong opponent that he thought he would be (for now), almost as if he was waiting for someone like that.
2. That in the end Luffy and the others still weren't willing to join his crew (i think this a call back for kaido for how the Rocks Pirates ended), that even if you have the strongest crew, if there was no unity and camaraderie, it would ultimately lead to disbandment of the crew.
And while kaido has shown to be cruel (like what he did to the people of Obokre town-although i believe that's more orochi's fault), Kaido has shown to have restraint and accommodation to the people around him and the people he likes. He allows Yamato's insolence, Ulti's brattiness, Jack's failures, and even feels sorrow for doffy's defeat.
Brannew said that Kaido became a yonko through pure strength along, and that people joined him because they admired his strength.
I think that what kaido really longs for is the return of the Rock's pirates strength. maybe that's also why he chose to ally with big mom.
but kaido doesn't have the level of charisma to attract people and allies like Luffy and Roger, or even Rocks D Xebec himself did. Maybe its because he isn't a D?
Why Kaido wants the One Piece to unleash chaos into the world is still a unanswered mystery. It must have something to do with his past that Oda still hasn't shown in a backstory yet. I wonder if it has anything to do when Kaido first joined the Rocks as an apprentice and took part in the attack on the World Nobles, and what he saw and experienced from that incident as a result. Which would be interesting because that's something that Luffy would also probably do in the future.
There is also an interesting part of Kaido's history that oda hasn't revealed yet and that is when Kaido was caught 17 times by the marines.
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