I agree with Never_Hesitate's assessment. The fact that advancement can only be achieved by killing your superior, is an unsustainable method of governance. This eventually leads Darth Bane to implement the Rule of Two.
Another issue you are probably not taking into account is that ALL sci-fi stories tend to have is a general lack of concrete passage of time.
There are several examples of this in the Star Wars universe. One of which is...
- the entire sequence of the Falcon evading the Empire in ESB:
- how long did the Falcon evade the Empire before entering the asteroid field?
- how long did the Falcon sit the belly of the asteroid beast before they figured out they were in the belly of an animal?
- how long was the sub-light journey from the asteroid belt (in the "no-oid" system to Bespin)?
[*}how long were they in Cloud City before Lando brought them to "dinner" with Darth Vader?
And during all this time Luke is on Dagobah with Yoda in Jedi training.
In movie time that whole sequence only took about 30 minutes, but it was probably months if not a year.
In this game the JK spends a year under the influence of the Emperor, but the time goes almost instantly for the player - there is a fade to black and fade back in.
And all the other class and planet stories happen in parallel. Chances are that several years pass during the leveling 1-50 process.
In addition, unlike in real life where many evil leaders build their "empires" over time and by the time the rest of the world realizes what is going on, it often too late...in SWTOR the Republic realizes what the Empire is doing and quickly swoops in to prevent it. Which is why the "war" is so short. The Sith Council is decimated and now the Empire has to start over again.
Lastly, as to how a smuggler or trooper can kill a Sith Council member like Darth Decimus - call it what you will, but in every story the hero has to defeat the villain eventually. Whether by luck or skill, the hero wins in the end.