King the Wildfire is an All-Star of the Beasts Pirates and one of Kaido’s three right-hand men, the Disasters.
Just going by the dude’s attire and the mention by Queen he is a pervert who loves torture, it’s apparent the dude used to work at Impel Down.
Naturally some have theorized that he is simply a relative of Magellan or maybe was a guard there like Shiliew of the Blackbeard Pirates. However I have a different idea. Specifically that once upon a time, King was Impel Down’s Warden, the guy who had the job before Magellan.
Even though Magellan is the Warden and probably had the job for a decent amount of time, there was obviously someone who held the position prior to him and considering the Bar set by Magellan would obviously be someone who’s insanely strong, like the first Commander of a Yonko Crew.
Of course some may argue that since Magellan is in his late forties, it’d be hard to believe someone older than him could be so powerful. Yet, One Piece has proven age doesn’t mean much, hell Big Mom is in her 60s and is a Yonko.
As to why King is not warden anymore… Well Queen mentions that he loves to torture people, something that sounds pretty similar to how Shiliew would just kill prisoners without care. It’s possible that King, since he’s brutal enough to be a member of Kaido’s crew and one of its top members, would get overzealous and torture prisoners to death, something Magellan, who we saw with Shiliew wouldn’t tolerate.
So he was probably replaced with Magellan, though since he likely would’ve been arrested or attempted arrest, he fled and joined Kaido instead.
This also plays into his dark wings, which make him look like a Fallen Angel, which going by the hell theme, it could also be argued he’s atleast partially inspired by Lucifer, someone who was once on the side of the just but fell from grace and became evil. Even his nickname “Wildfire” leans into this since Hell is often associated with Fire.
One other thing, it’s possible that he might have helped come up with some of the tortures that are found in Impel Down since the dude loves to torment people. After all, someone had to be insane enough to come up with those brutal practices and this guy certainly seems to fit the bill.
*Theory by WII_DJoker