In TOR you get a quest as the Empire to gas some revolting slaves. They Sith giving you the quest wants you to try out his special gas that cripples the slaves with unbearable pain. He says its to encourage them to surrender quicker but you learn that all previous experiments point to there being no actual increased surrender time. So basically he's just torturing them for no reason. In fact, its actually extremely ineffective to the Empire and a waste of time and resources.
Another Sith Lord plans to bomb the Empires own citizens from orbit to create an Empire of fear and pain. Why? Because he's ****ing nuts.
The Sith are almost by default insane murderous fascists. The darkside has been described by Lucas as being like a cancer in the Force. It amplifies the most destructive emotions in its users and basically drives them mad. The Jedi don't just hate them for being a different religious organization, they hate them for being an organisation of genocidal lunatics whose stated goal is to wipe out the Jedi Order and enslave the galaxy.