Even Darth Tyranus said, that mercy is useful sometimes. Usefulness is also very important stuff in Sith ways. When you making own power structures, you just MUST be like Jedi, find knowledge, make deals in shadows, controlling own passions. Irony.
So real good Sith is like shapeshifter. But most of them are inevitably consumed by the Dark Side and destroy by it. Who knows how good ruler would've Palpatine been, if he didnt turn more mad and ugly from own lightnings?
I thought that earlier with someone else. The morality stands, Sith have none, Jedi bend easy with it sometimes