Law, while taking his leave from the SAD room, blatantly said, "Pirate Vergo".
I think there's more to this than meets the eye.
Plus just a few panels earlier, Vergo said "You don't know Joker's past".
So, "Joker's past" + "Pirate Vergo" = Vergo used to be a pirate until Doflamingo pulled his strings (coincidentally is his DF power?! xD) in the WG / Marines to put him into the marine ranks.
Blast from past : Who was it that was specifically choosen by direct instruction from WG / higher marine ranks to kill a certain incompetent Shichibukai? Its Doflamingo... coincidentally?
Let me know your thoughts.