Good point bringing up Caligula and Roman History!
However, I think the situation is more like Catalina in that the traitor doesn't like the country's direction, wants to change it, and wants to take over with his plans and ideas at the head of the country. Fury sees where the world is headed- he is preparing for the war with his own superteam.
Then again, it could be Cap, who sees that Fury is indeed doing this, and wants to stop Fury from doing what he is doing, as we saw Cap isn't too happy with Fury in Ultimate Six. (see Leather_wolf's signature right below this post) He seemed pretty set on stopping Fury at some point with his expressions and dialogue. He just doesnt like what Fury's doing.
Now that I think about it, I think Millar is going to throw us a twist: It was Cap who is the traitor, but he is going against Fury and his policies.
Yeah, this works!!!:
Cap has Jan and Hawkeye on his side, thinks he has Natasha, Jan or someone visits Hank, Fury gets word from Natasha or Jarvis or Son of Satan or someone what Cap is doing, so he sets out to take out Hawkeye and Cap's "Avengers", i.e. Fury kills Hawkeye. So it wouldn't be the traitor who killed Hawkeye. Fury woulda also "killed" Banner, taken out Thor, i.e. phasing out the other people, and now he has to take action against the "traitor," the one who is staying true to American beliefs, Cap'n America. Stark will have to choose sides, Fury will have an army to fight his war, and Cap and the Avengers will be "traitors" and try to stop him. And Loki would have allied himself with the "real traitor," something quite "powerful," i.e. all of SHIELD.
The title of Traitor is all about point of view.
That's it, I've figured it out! Eureka!
Anywho, Caligula was just a crazy old coot.