King's race is ancient and has some sort of connection to the taboo topics the WG continuously hides and denies, earning them a Buster Call that ended their lives, with only King surviving.

Perhaps King goes on to have revenge on the WG, either alone or with Kaido's assistance, and absolutely annihilates and burns to the ground everything in his path towards the Vice-admirals that led the call, earning him his title.

Maybe King's (and his tribe's) home island was in God's Valley, leading to a meeting with Kaido on "that fateful day" where Kaido met and saved him, becoming Kaido's first mate and starting his new crew.

King's design is also appropriate for this scenario. We know barely anything about his actual appearance under his leather clothing that covers him almost completely. His most distinctive traits are his large, black wings and the fire out of his back. The spikes around his head are perhaps natural and not part of his suit. These traits are demonic in nature, so it'd be a pretty rich irony if his demon-like tribe resided on a place named after God. This island is also characterized by tall cliffs and canyons, giving it plenty of verticality fitting with the fact that King's tribe possess large wings, but it's also possible he can't use his wings at all, as we've only seen King properly fly in his full-Zoan mode (During his clash with Sanji, it remains unclear to me if King simply jumped or was using his wings for proper, actual flight).

This could reinforce them even further as demons, as they'd be fallen angels with their ability to fly taken from them.

I got a final thought after this last bit. What if the "fallen angel" concept is correct and refers to the tribe being removed from their actual home, which is Mary Geoise, forcing them to take residence somewhere else much like an angel being removed from heaven and sent to hell? Being the original residents from the place that could be called heaven in the One Piece world, it's only natural their new home is named (likely self-named by its now new inhabitants) "God's Valley" as they literally were the former gods of the world before being wrongfully removed (during Void Century, likely).

Credits- Walixen