There are various perspectives and opinions on what makes a Dark Jedi. For example, you have the Legions of Lettow led by Xendor, the first Jedi that fell to the dark side who caused one of the biggest Force Wars that hadn't been heard of in the Jedi Order's history since the Je'daii split on Tython. It is the same for the second Dark Jedi exiles, Ajunta Pall, Sorzus Syn and so on who fell to the dark side and manifested in ways that only few, few Sith in the Sith Order's history have managed to replicate and build on.
On the other hand, you have the term that is "Dark Jedi" who were Force-sensitives that were not Jedi (light siders) but neither were they Sith. They acted as servants for the Sith, acting as their operatives, spies or basic servitude. There were then those Dark Jedi who served the Sith who were pretty powerful. Take the Dark Acolytes that served Darth Tyranus and then later Darth Sidious, whom eventually became the Emperor's Hands. It was an servant that came out of the Rule of Two, called Lumiya, that saved the Order of Sith Lords that fragmenting altogether, only for Caedus to fall to his twin sister.
It is my intention to roleplay a character who has begun to think beyond the realms of the Sith. Essentially a dark sider who has moved on from the Sith religion and is no longer tied down by it's beliefs or dogmas, instead focusing on what it truly is to be dark sided. Key differences between Vilox Pazela and Darth Sidious, for example, is that one knows when to step back and the other does not. If someone crossed Sidious, then that Sith Lord would create a complicated, highly intelligent plot to gain revenge on that one particular individual. Vilox Pazela, on the other hand, would remember that individual but wouldn't be blinded by revenge nor his hatred for that individual. He would know that, whatever plot he was making at the time, would eventually affect that particular person and eventually allow him to orchestrate and manipulate events to place that person in a situation that would equally messed them up or worse, in return for what they had done
several years ago, rather than what I think a Sith would do, who would have the patience to wait some years, but not as long as someone as Vilox Pazela.
In my other community, I would drift in between the Sith and the Dark Jedi, whenever a roleplayer or two would like to spice up our relatively small Star Wars world and begin something new with the Dark Jedi. In the end, character development dictated that Vilox Pazela became the Dark Lord of the Sith Darth Vazela. But whenever he was affiliated to the Dark Jedi as Vilox Pazela (and let's not forget, I started the character as a Dark Jedi) it was always fun to portray and roleplay something new, with a different way of doing and seeing things without the influence of Sith teachings, but equally or sometimes more damaging than what a Sith did.
As I once described it, there were three different types of Dark Jedi:
- The servant, whom serves a master always. A pawn for their master to utilize in their plots, without much reward save for the fact that they were serving that person.
- The isolationist, whom cares about nothing or no one but themselves, who doesn't have the aspirations or objectives that a Sith Lord would have, but not necessarily weaker than a Sith would be. A dark side artifact hunter, for example.
- The exile, a Dark Jedi whom is a master of the dark side. Who is as powerful and as objective as a Sith, but with a different style, codex and perspective to the Sith. The Lettows of Legion or the exiled Dark Jedi to Korriban, for example.
I've roleplayed a bit of all three when I've played a Dark Jedi, but character development always ended in the latter and that was always fun.
, I have a plot in the works 
