I’ll try to explain Kaido’s character and what led him to where he is today.
Starting from the beginning, we can see that Kaido is 10 years old. A boy who appears to have been born on the battlefield and the only thing he knows is to fight and is hungry for more.
At the age of 13, he was sold by his country to the government to become a Marine (the same thing would happen to Big Mom). He says he’s not a bargaining chip and doesn’t want to become a government dog. His country did so in order to continue participating in Reverie.
Kaido escapes from the naval ship and is said to have allowed himself to be caught to eat. This is probably where the part of Kaido being caught 18 times came from.
When Kaido turns 15 and at the request of the Rocks, Whitebeard asks if he wants to become a pirate and join the crew. Kaido accepts.
At the age of 21, the God Valley incident occurs. From the dialogue, it looks like there are some opportunities for the Rocks to make something happen. Garp is said to have defeated the rocks (Roger’s name is not spoken by the pirates at the base of the rocks). After this moment, we only see Big Mom worried about looking for Kaido. Big Mom may have given the fruit to Kaido so much that he joined her crew (the oni race) and escaped without a trace (hence the conflict between the two).
This information is only from chapter 1049… but something he doesn’t show is the ten years that have passed since the God Valley Incident until the time when the old witch went to talk to Kaido. However, there is a bit of a flashback in Chapter 1035. In this chapter, he tells Kaido King that he’s the only one who can change the world. It seemed that they had already spoken before this moment. Upon leaving, they appear to be in Punk Hazard’s lab and Kaido calls him the King.
Next comes a flashback again in Chapter 1036 where Kaido asks the King if he thinks he’s Joey Boy. And here’s the twist, it’s not Kaido who’s looking for a Joy Boy, but a King. King asked in Chapter 1035 if Kaido can change the world, because he is the one waiting for Joy Boy (the one who will change the world). In Chapter 1036, King says that legends are legends, which means that he no longer cares about Joy Boy, because how Kaido saved his life (other than his world?) Joy Boy no longer cares about him. Kaido never wanted to be a Joy Boy either, he just asked King if he would be the number the Lunarians were waiting for.
Now we come back to Chapter 1049, where the old witch suggests something to Kaido (going to Wano). Also in this section we see Kaido’s true desire. He wants a big war (as he said in his first appearance) involving the whole world. It is only in war, he adds, that the worth of each person can be seen. Kaido is a character who believes in the law of the strongest, in the idea of the king of the jungle, the king of beasts as his name says. After noticing the rotting of nobles when he was young (selling to worse trash) and perhaps noticing something in God’s Valley (protection from celestial dragons, weak and corrupt beings that are literally the opposite of Kaido’s mindset), he came up with this idea of a world where only the strong stand over the weak. It’s a nice similarity to what the law says to Tashigi in Punk Hazard.