but shouldn't Sabo be member of the Revolutionaries crew?! at least we know that Dragon saved him in the flashback…
Not necessarily. Probably not every person out of the who knows ow many people the revolutionaries have saved, has joined the revolutionaries.
Sabo could have abandoned his altruistic goal in exchange for a villainous one, maybe out of practicality. Or maybe Don Flamingo is a means to an end, and Sabo's just using him. Sabo could also still be part of the revolutionaries, acting as an _sabo_teur. Assuming it is Sabo at all, though the hat is noteworthy.