Okay, this may be a bit of a stupid question but: Why did Doflamingo conquer Dressrosa the way he did in the first place? He may have given up on his title as a World Noble by becoming a pirate but due to his status as a Shichibukai he obviously got back all his rights and privileges. So why the hell was it necessary to overthrow the Riku Family and kill a bunch of people? Why didn't he just waltz into Dressrosa like "Surprise, b*tches! I'm your new King because my old man wore a wacky space suit and my family ruled this place long before you were old enough to shit your diapers."
I guess he wouldn't have been as popular this way but wouldn't that have made everything a lot easier?