This flashback is so underrated.
We literally see that Kaido used to be like Luffy. He freed an innocent kid being tortured by the WG, and treated him with respect and kindness.
Something had to happen to change this. Even after Rocks, Kaido wasn't evil; just misguided, since we know he saved King after Rocks. We are missing something. Kaido saw something in his life that changed him from the kind warrior who saved King out of kindness, to the monster than was willing to enslave an entire country.
Between the five years of escaping Vegapunk's lab and saving King, and being convinced by Higurashi, the Kurozumi Hag, to invade Wano, something had to go horribly wrong in Kaido's life.
Maybe King had knowledge from his Lunarian linage that proved Kaido would not be Joy Boy?
When I see Kaido's old smile, it reminds me of Luffy and Zoro. Kaido was hopeless before joining Rocks, then, gained a lot of confidence after God Valley. We never see Kaido smiling in the past, really, only in the King flashback.
Something had to have happened on God Valley that inspired Kaido, and then something happened that broke than inspiration.