This the sith philosphy
Sith code
Peace is a lie; there is only passion.
Through passion, I gain strength.
Through strength, I gain power.
Through power, I gain victory.
Through victory, my chains are broken.
The Force shall set me free.
The dark side of the Force is innately tied to the distinctly negative ethical paradigm of the Sith. It is largely based on emotions and passion rather than peace and serenity which are preached by the light side of the Force. The dark side of the Force comes from the hate, anger, fear, aggression, vengeance, and malice in all living things. In the Star Wars movies, the practitioners of the dark side are Darth Sidious and his successive apprentices Darth Maul, Darth Tyranus, and Darth Vader.
One might compare the Sith code to Existentialism, as many aspects such as the quest for passion and freedom, and the concept of defining one's own destiny, are shared by the two. Other similarities between the two beliefs include awareness of death (the Jedi do not believe in death, they believe there is only the Force) and the absence of peace due to the role emotions play in life. Existentialists focus on angst and despair as major emotions, and these have been shown as aspects that help define Sith lords such as Anakin Skywalker).
The Jedi believe that emotions such as fear, uncertainty, doubt, anger, suffering, and attachment lead to the dark side. The Sith, on the other hand, believe that such emotions can be used to fuel one's own will to power, ego, and selfishness. The destructive havoc caused by the actions of a dark side practitioner are excused as serving a greater purpose under this philosophy.
The negative emotions increase the strength and abilities of a dark side practitioner. As a result, the dark side of the Force is extremely addictive: every time one calls on its power, one becomes more and more attached to it. Darth Vader desired to use this to his advantage during his time trying to turn his son, Luke Skywalker, to the dark side. If he could convince the young Jedi hopeful to call on the dark side enough times, the boy would find it more difficult to resist in the future. The Dark Side can also strongly affect the user physically, as the intense emotion and rage required to fuel it usually corrupts the user's body; this is demonstrated by Darth Sidious, Darth Vader, Darth Traya, a dark-sided Jedi Exile, and in extreme cases, Darth Sion.
The corruption also extends to the personality and soul of the Sith, as the emotions called upon eventually become the only emotions the Sith truly feel. Eventually the original purpose for using the Dark Side is forgotten as the emotions drive them to seek absolute power. The soul also suffers, as in death the emotions burn out and leave them empty. This is seen in Ajunta Pall, the first Dark Lord of the Sith who, even centuries after his death, lingered in his tomb, unable to join the Force because of the evil he caused and his inability to forgive himself.
The Sith also believe that instead of living alongside the Force, one must master it and use it as a tool. This is shown in the novelization of Revenge of the Sith, on page 72:
"They allowed the Force to direct them; Dooku directed the Force."
Overall, dark side relationships and organizations are inherently unstable. For instance, the pupil-master relationship of the Jedi is perverted under the Sith, as both the apprentice and the master naturally plot against one other, and one will eventually kill the other. For the Sith, this in-fighting is a positive: it guarantees that the strongest (and thus most fit) will rule. However, every time a Dark Side Force has risen in the galaxy, it has collapsed from inevitable instability due to this practice. The Empire is no exception. When Emperor Palpatine and Darth Vader discovered that Luke Skywalker was Vader's son, both wanted to turn Luke to the dark side. Vader wanted his son to help him kill Palpatine and rule the galaxy together, but Palpatine wanted Luke to take Vader's place as his apprentice.
This phenomenon of constant internecine warfare indicates that the Dark Side may in fact be weaker than the Light Side, since even by the Sith's own beliefs their inability to hold power marks their rule as unworthy. This instability due to in-fighting is in line with George Lucas' characterization of the Dark Side as "cancer" and is likewise one way of arguing that evil is weaker than good in Abrahamic religions; it was no doubt seized upon by proponents of the Potentium theory of the Force (see below).
Darth Bane considered the Force to be finite, and thus best restricted (ideally) to the use of a single Sith Lord. After the Battle of Ruusan, he eliminated the practice of having vast numbers of Sith at the same time and concentrated the dark side of the Force into two individuals: a Sith Master and Apprentice. This tradition, or "rule of two," remained in practice throughout the Star Wars Extended Universe. One might argue that the corrupted Jerec was an exception, as he had seven apprentices/fellow Dark Jedi on his command when he sought the power of the Valley of the Jedi, but Jerec was not a Sith, but rather, a Dark Jedi.
The Dark Lords Sidious, Tyranus, and Vader had each trained some followers with dark side abilities and occasionally inducted a member deeply into Sith lore and powers, but none of these followers were given the full Sith rites and training; thus the "rule of two" was consistent during the era of Darth Sidious.
