Wether Saurfang or Sylvanas is the traitor is all purely opinion based. We have the story so far, and we know whats happening. That is why Blizzard is doing this player agency thing with this. You can choose who to follow. The ones who will follow Sylvanas because they think she is worthy as Warchief, is just as justified as those who follow Saurfang because they think the Horde is going in the wrong direction.
So "Whos the traitor" will never really be stated as fact with the story we got so far. It's all up to each individual player to figure out whos the traitor or not. People can discuss their opinion as much as they want to, but don't pretend it is the fact just because YOU think it is.