Ever since we found he was working with BB after Dressrosa it was so obvious to everyone that he was probably scheming against BB but once we got the Hachinosu chapters awhile back it convinced that he truly serves under Blackbeard not because he’s so loyal to BB but because it’s something he decided for himself which to me is much more interesting then him being undercover
I think he has his own personal motivations and justice he wants to uphold, but is not a part of the Marines anymore. He may very well betray Blackbeard, but not to rejoin or work for the Marines.
I don’t think he actually fully went over to the pirates side but i don’t think he’s an undercover marine either. I think he has his own intentions. He’s definitely up to something.