The Sith Order of this day and age was built upon the ideas and philosophies of the ancient Sith Empires; the philosophies of Darth Sadow and Darth Vitiate and their predecessors. Unlike their Jedi antitheses, the Sith way is to embrace instinct, passion, and primal emotions and impulses. Like their ancient Sith idols, the current Sith Order believes that conflict, whether internal or external, is what defines an individual. Conflict challenges both their civilization and their culture, forcing them to change and adapt quicker than their Jedi foes.
It if for this reason that the Sith are able to change tactics and methods with each incarnation of their Order, allowing them to combat the Jedi on different fronts and in different manners. The Sith's power is wrought from anger, hatred, aggression, unbridled passion, fear, covetousness, cruelty, selfishness, jealously, and malevolence. These fundamental beliefs of the Sith Order are reflected in the Three Creeds; the foundational philosophies that dominate the Sith way of life and thinking.