That is still three people who make heavy use of the dark side and the rest which must have at least some experience with it else they would have no ability to learn Vectivus' management techniques. All this with practically no institutional knowledge and making stuff up as we go along. Not exactly 'once you start down the dark path forever will it dominate your destiny' is it?
You are ignoring one, critical fact.
The Abyss Walkers are not Sith. Neither is Ciaran, Asajj and, perhaps, Dani..
Using the Dark Side
does not make you a Sith. The Sith are a
culture. They follow a very specific code of behaviour, have art and traditions of their own, and the Dark Side is merely an aspect - however important it might be - of said culture. Ciaran is Darth Traya; the very definition of that title is one who "betrayed" the teachings of the Jedi and the Sith, which, by definition, makes it impossible for Ciaran to be a Sith. Asajj is the modern exile, who abandoned the teachings of the Sith and was abandoned by them in turn, so that she could make her own way. Dani... well, I don't know anything about her, so I can't tell.
The point is, the Abyss Walkers are not Sith, because they do not subscribe to the culture or the code, and merely use the Dark Side. At the very most, that makes them Dark Jedi, or just Dark Force Sensitives. Furthermore, the critical difference between the Sith and the Abyss Walkers is the fact that the former are obsessed with the Dark Side and are willing to do anything to increase its power, which in turn increases its grasp on them, because it feeds on dark emotions and demands deeds that force you to feel more dark emotions. For example, those who fully fall to the Dark Side often end up killing their loved ones, because even if they went Dark to save those same people, they mysteriously start to see them as weaknesses that need to be removed, or kill them in a fit of uncontrollable rage that, curiously, they never experienced before they went Dark, which tends to result in self-loathing, guilt and hatred, which makes them use more Dark Side, which makes them kill more people... etc. The Sith also follow a code and a culture that makes this not only
acceptable, but
right.
That makes them all Sith. The Abyss Walkers, on the other hand, don't follow any codes or cultures, be it Sith or Jedi, and instead see the Force as a giant box of tools, some colored lighter than others, and only use whatever is necessary for the given situation. I absolutely refuse to call them "Grey" because virtually every single "Grey" character I've seen in the Star Wars fandom was actually a glorified Sith in disguise, except written to be much cooler and special, because he could use the Dark and the Light in his righteous quest to achieve balance by... doing what the Sith do, and establishing their own
Sith Empire on the burning ruins of the Republic, right after slaughtering the Jedi, but I suppose "Grey" is the closest thing to the way they use the Force. I'd prefer to just call it the "I Don't Give A Fuck About Your Shades", though. Because the Abyss Walkers do not rely on the Dark Side alone, and do not follow a set of beliefs that demands they focus completely on it and use only negative emotions for power with very little restraint, they do not enter the loop of dark emotions that corrupts people. Well, except for Dani who, judging by what the other posters wrote, is the only member of the Abyss Walkers who
did enter that spiral (or is just plain insane)... and despite not being a Sith (unless she follows the Code and the culture), is on the verge of going Cacklefuck "Unlimited Powah" McGenocide.
"Once you start down the dark path forever will it dominate your destiny," refers not to using the Dark Side (why, if I remember correctly, Mace Windu's lightsaber form uses it, and he's a member of the Council), but to, again, following the code and teachings that demand you cut away all bonds and positive feelings, so that you can replace them with negative ones. When you go down
that dark path, unless someone pulls you out or you notice the way your common sense is rapidly dying, you will almost inevitably
Fall to it, so intoxicated on the power, focusing on it and drowning in it to the point you never bother looking for a solution that
doesn't use the Dark Side. Lacking anyone to pull you out or restrain you, because your teachings demanded that you cut all "weaknesses" that are your bonds... that is a stage that the Sith, due to their culture and code, almost inevitably reach at one point. At that level, you get Kylo Ren killing Han, you get Anakin choking the love of his life, for whom he went Dark in the first place, and massacring children...and at point, you also get Darth Malgus murdering his lover, Eleena Daru, to eliminate his "weakness", because he allowed his feelings towards her to influence his actions (oh, the hypocrisy). Once you reach
that level, any thought of the "Light" side, or any positive feelings, become utterly repulsive for you, because it keeps reminding you of everything you have thrown away and destroyed for power... which is just as well, because that is also the point at which the Dark Side mutates you so much that it would take you at least one miracle to inspire any positive feelings in another person.
That is what going down the dark path means.
That is what being a Sith means. And let me tell you, not a single individual in the Abyss Walkers is a Sith... barring the possible exception of Dani. I don't know, because I don't feel like looking through the thread for her detailed description.