The false traitor is probably: Kyoshiro, one of the nine scabbard members revealed/spoiled from the new One Piece opening.
The new one piece opening (Begining from the 38 second):
I think he was waiting 20 years after in order to take action agains't the Shogun and Kaido, the goal is to get infos internally and send them in a false way when the day arrive for the raid... It was the right moment to play his card.
This 2 panels from Chapter 944 confirm Kyoshiro more as an Ally (Kyoshiro/Hiyori looking at each other):
He helped Hiyori with fake death.