Hmmmm, the Jedi always talk about the dark side being the 'easy path' to power, but maybe there's more to it than that. Maybe you don't need to use strong emotions to tap into the dark side's power and that through enough concentration and strength of will you can just call upon it like a Jedi calls upon the light side of the Force. The difference here would be that instead of being a weapon of the Force, the Force becomes your weapon, because you have learned to control its power rather than let it control you through 'giving in' to your emotions. The Sith Emperor after all says:
Traya most probably focused anger and that is why she remained calm, being in state of anger doesnt exactly mean to mumble and throw shouts. Althou I doubt that any experienced Sith would become furious easily, after all they slay many different beings in hundreds, it must have become a routine. Of course, enemy who would put equal fight probably would enrage a Sith and then it is a matter of a discipline.
Personally I'm much more interested in the Emperor, Lord Scourge and Sith Warrior as they do not possess emotions any longer, for them I guess same presence of the Dark Side inside them is enough to channel it in combat or they focus on murderous intent.and thats how it work. I would say that they are the ultimate form of the force sensitive fighter.
There is no death, there is only the Force. And I am its master.
And that guy seems pretty devoid of strong emotion, but then again he does seem to use anger in his battle with Revan if I recall. Maybe if you just learn to focus your emotions enough you eventually learn to control them, and so can call upon the essence of anger, fear and hatred whenever you want without actually having to feel those emotions at the time, just a thought.