what makes you think that?
Firstly, keeping him so mysterious for this long only for him to turn up as a good guy would be a waste.
Secondly, introducing the main villain in the first chapter itself seems like something Oda would do. Also, Oda wanted One Piece to be only 5 years long at the start so it's highly probable that he wanted the main characters, antagonists included to be introduced early on.
Thirdly, his connections to the WG like with Sengoku saying that he's doing him a favor as it's 'him' and the Gorosei saying the same thing. Also, his very probable connections to the underworld.
Lastly, I have various different theories on how Oda can go about him as a villain, a lot of which are connected to Rocks himself. I think I'll share them later in detail.
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