The problem with Thaddeus's betrayal is that the characters whine about how he's "so young to be a Space Marine" and for, like, 4 straight dialogue chats, Thaddeus acts like a punish loner teenager repeatedly crying "you don't understand! You weren't there, you don't know what it was like! /wrist"
Cyrus's betrayal was my favorite reason, but ultimately, Tarkus and Jonah are the most likely traitors. However, since we heard Jonah's voice in the Retribution Imperium vs. Chaos trailer, it MIGHT not be him.
People say Avitus should be the traitor because he was too bloody, but he HATES traitors and turncoats. I doubt it was him.
Force Commander could not have been the traitor. That's stupid. *sigh* However, I wouldn't put it past Relic, as all the other characters had voices and faces, whereas Eff Sea Vanilla Ice was just a face. Easy to replace.
Tarkus, because, well, Tarkus. The most DELICIOUS traitor. No other character has items detailing their descent into Chaos. It must (read: it should) be Tarkus.