There is no peace, there is anger
There is no fear, there is power
There is no death, there is immortality
There is no weakness, there is the Dark Side.
I am the heart of darkness
I know no fear
But rather I instil it in my enemies
I am the destroyer of worlds
I know the power of the Dark Side
I am the fire of hate.
All the universe bows before me
I pledge myself to the darkness
For I have found true life
In the death of the light.
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 free me.
The ideology of the Sith has changed with each rise and fall of the different Sith Lords that have claimed rulership. Since the founding of the Sith Dark Alliance, however, there has been little change in what the Sith beleive, or their goals.
Thousands of years ago, the Sith were confined to the Rule of Two, as set out by Darth Bane. It was effective for when it was in place, as the Sith had not only been self destructive, but were also nearly wiped out by the Jedi and Republic forces. As the centruries went on, they stayed hidden. The Rule of Two stayed in place for several centuries. That all changed around 4,900 ABY.
After 4,900 ABY, the Sith realized that they were becoming numerous once again, and that the only way to truly defeat the Jedi were to combine their forces. In 4980 ABY they formed a pact among the more powerful Sith Lords, and began to attack the Jedi in unison. Seeing their victories mount, the Sith Lords decided to do away with the Rule of Two.
Even after the collapse of the Dark Brotherhood of the Sith, they held to the belief that there was more strength in numbers. They also changed their view of usurping the power of their masters. It has become a rare occurence where an apprentice actually kills their master. While the Sith High Council rarely steps in when such a thing happens, they do an investigation as to the circumstances. If the Sith Master is deemed to have been weak, and deserving to die, then they will do nothing. If, however, the Master was deemed still a worthy ally, and no good cause can be found for his death, the one-time apprentice is usually sought out by the Council, and brought before them to answer for what they have done. There has only been one instance when the accused has been allowed to live. The ritual for determining their worth is a closely guarded secret.
The Sith still seek to control the galaxy, as they always have. They have, with the help of the Mandalorians, nearly destroyed the entire Jedi way of life from the galaxy.