I'm starting to believe that all these flashback chapters we've had of Oden's history and the scabbards directly confirms that the traitor in the alliance is indeed, one of the red scabbards..
I mean, think about it... These chapters allowed us to explore in-depth on how each of the scabbards met Oden, how much they loved him and how much they devoted their lives to him.. These chapters are perfect in distracting us into thinking it can't possibly be one of the red scabbards, when in fact, it is.
I think Oda is using these flashback chapters as a perfect setup tool, as a way to deliver a big shock to us when he finally reveals who the traitor is, when the scenario points to the present again.. a red scabbard being the traitor is the perfect shock that we would have, after all these chapters that revealed their loyalty to Oden.
A potential question I'd like to open up here is: the potential of the traitor in the alliance been against Oden. Would he be one of the biggest snakes in the series? Has he been against Oden from day one?
Not necessarily so. Whoever the traitor is - perhaps he loved Oden too, as much as the other scabbards, and was loyal to him.. he probably desired revenge as much as the others, but gave up with the realization of how hopeless it utterly seemed to take Kaido down, when even Oden couldn't do it; when even all the samurai that Wano beamed with pride 20 years ago, couldn't take down Kaido.
The traitor - I believe - thinks this plan will fail, like every other plan.. and has betrayed the alliance for the reason that he doesn't want more innocent lives lost, and has already accepted Wano's declining fate.