General QJS/Grievous may not be a good example to use, given recent Star Wars literature has been showing him more as a brain-altered slave (he originally was a war leader fighting off an insectoid race known as the Yam'rii who were trying to enslave his people, then later went "Hiroshima" on the Yam'rii and was offered some unknown benefit by Dooku, who blew up his shuttle, San Hill tortured him electronically and then they installed alterations in the anger part of his brain and the memory part) than a malevolent entity or person like Sidious or Dooku. Furthermore, he exists in a Galaxy where A. Ontological good and evil (Light side and dark side of the Force) are shown to clearly exist in many species (There have been nonhuman Sith and Dark Jedi, such as Desann, Jerec, Darth Rivan, Darth Maul, a Chagrian Sith from the Legacy comics who's name escapes me, a Twi'Lek Sith from these same comics and even Darth Plagueis) B. Cultures generally understand each other (There's Ithorians, Twi'Leks, Rodians and the like in the Senate, and even obscure species like the Chiss or Yoda's race exist in prominent positions in Galactic society).
We don't yet know the reasons the SK destroyed Sol. Unless they are irrational and waste resources blowing up stars for no reason, they are likely to have one. There is also a possibility the SK that showed up in Sol are not the ones we see in Sirius, since Quintane's speech opens the possibility to them in other sectors of the DK Empire and developer documents indicate they had rosen against humans for their waking up of them.
My argument there is largely a caution against applying human thought or morality to extraterrestrial races. They are not totally inapplicable, but caution should be applied regardless. The Cylons are machines built by humans, whereas the SK are the genetic creations of an even more mysterious and power extraterrestrial race. Especially when you consider the fact SK are pleisiosaurine, not humanoid.