All the Sith Lords in KotoR 2, along with Revan.
Nihilus and Sion were victims of the horrors of the Mandalorian War. They were wounded in such a way that their addiction to the power of the Dark Side was the only thing that kept them alive. Sion was left with nothing but pain and it was the only thing that sustained him. Nihilus became less of a man and more of a gaping maw, existing only to feed upon life.
Kreia sees the Force as the source behind much of the horrors inflicted upon the galaxy. Countless holy wars are waged in the name of religious dogmas based on the light and dark side of the Force. Billions of lives are sacrificed to maintain balance in the Force. Kreia's goal was to introduce a new set of beliefs without the need for the Force. She wanted to teach people that the power of the Force comes at a price, that one can walk away from such power and be stronger for it, and that life can exist without the Force.
Revan of course, saw the threat of the True Sith. He realized how the True Sith orchestrated the Mandalorian War to reveal the hypocrisy within the Republic (and the Jedi). The Core worlds were slow to respond to the Mandalorian invasion of the Rim worlds and they did not mount a real defense until they were threatened. As a result, faith in the Republic itself was shaken. Republic worlds were no longer willing to fight to defend the Republic or its principles since their so called allies won't fight for them. Revan sacrificed himself to the dark side so that he could either unite the galaxy under his own banner, or he would remind the Republic and the Jedi of the principles that they stand for, have their beliefs reaffirmed once more through the war, and unite to defeat him, growing stronger and wiser through their victory.
