I don't want Byakuya to be a traitor. After his almost fanatical devotion to killing Rukia because Central 46 supposedly ordered her execution, I don't believe he would change so much as to turn traitor in the space of just a few months.

Eh, at first I interpreted that scene as showing that Senbonzakura is as an aloof a bastard as his master. Recent discussions with a friend have made me think that not just Senbonzakura, but also Katen Kyoukotsu, Sougyo no Kotowari and Minazuki are playing Muramasa's little game only as far as they wish to, perhaps with Byakuya's knowledge and assistance in Senbonzakura's case.