Reread chapter 603, and the whole “Dark King” epithet has an interesting reading if Rayleigh was actually a king.
When Roger first meets him all Rayleigh says about his past is that his home burned down and he stole this boat and lives in it.
Another fun visual clue is that Rayleigh is not in Loguetown’s docks, at least not the one the Straw Hats were in, and that’s Roger’s hometown.
So in the manga Roger just shows up, asks his name, and Rayleigh is hesitant and just says “…Rayleigh” and then Roger says well this is a fortunate occurrence, how’d you like to turn the world upside down with me?
And that’s where the manga flashback ends and returns to Rayleigh tearing up.
What if “Dark King” Rayleigh, is quite literal. The World Government didn’t like him, said he was a dark king, probably for not joining the World Government, and liberated the kingdom from Rayleigh and burned his castle down.
So he just fled on this boat, planned to drink away his days knowing the World Government was chasing him on the run, and suddenly Roger gave him a chance of turning the world upside down, and he figured why not? I’m gonna be killed anyway sooner or later might as well become a Pirate with this guy and piss off the World Government as much as possible.
*Theory by MaimedJester