Aside from formidable Star Wars villains like Darth Vader and Emperor Palpatine, the Jedi-hunting group known as the Inquisitors are quite strong in the dark side of the Force; yet throughout Star Wars, the Inquisitors successfully avoided one important Sith rule. Upon their initial introduction in the Star Wars Rebels animated series, the Inquisitors were confused by fans to be Sith lords right off the bat. However, as the series went on and more information about the group was divulged, their differences from those of the Sith Order became rather clear thanks to one particular aspect of the Sith code.
What really sets the Inquisitors apart is their failure to adhere to the Sith's most important custom, the Rule of Two. The Rule of Two was a very strict Sith law created by the ancient Sith Lord, Darth Bane, who was the sole survivor of a Sith massacre about 1000 years before the Clone Wars took place. As a means of getting revenge on the Jedi Order, as well as a way of rebuilding the Sith, Bane created a rule that only two Sith lords can exist at a time, one master and one apprentice. If the apprentice wished to become a master themselves and take up their own protégé, they were required to strike down their master and take their place at the head of the order. As a result, the Sith Order would grow more powerful with each generation while remaining in the shadows, which was all part of Bane's plan to destroy the Jedi once and for all. By contrast, the Inquisitors were exempt from the Rule of Two, partly to protect the Sith code, but also to avoid the infighting that had led to their pre-Bane downfall.
Why Inquisitors Aren't Sith In Star Wars
It is a common misconception that the Inquisitors are actual Sith lords. Admittedly, they do utilize the dark side, fight with aggression and anger, and even wield red-colored lightsabers, the weapons of choice for most Sith. However, unlike the more rounded Sith lords, Inquisitors are Jedi-hunters first and foremost. After the enactment of Order 66 in Star Wars: Episode III - Revenge of the Sith, the Imperial Inquisitorius Program was formed by Emperor Palpatine in order to track down and kill any remaining Jedi in the galaxy that managed to survive the initial purge. As such, the Inquisitors found their strength not only in the dark side, but also in their numbers, which were used to their advantage when it came to hunting Jedi.
While they may have been brutally trained by Darth Vader himself and allied with Palpatine, they are not truly Sith as the Rule of Two's roles of master and apprentice were already filled by the two. This makes the Inquisitors themselves glorified tools that were used to further the goals of the actual existing Sith lords. As such, while they were impressive fighters in their own right, the Empire's Inquisitors were no match for a fully-trained Sith.
Star Wars Already Broke The Sith Rule Of Two
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While the Sith's Rule of Two was not technically broken by the Inquisitors seeing as they are not members of the Sith, it has in fact been broken many times throughout Star Wars canon and legends. The most obvious breaker of the Rule would be Emperor Palpatine, as he has had three (if not more) apprentices over the course of the three trilogies; his most notable students being Darth Maul, Count Dooku/Darth Tyranus, and Anakin Skywalker a.k.a. Darth Vader. As Maul was his first pupil and thus did not break the Rule, both Dooku and Anakin were both willingly/unknowingly doing Palpatine's bidding before his current apprentice was disposed of, not to mention the fact that the two also secretly trained their own apprentices for the sole purpose of having help in killing their master, Palpatine (which ultimately failed). This all goes to show that, although the Rule of Two was not broken in regard to the creation of the Inquisitors, it can and has been broken several times over the course of the series.