Not sure I agree, Dark siders might temper their sadism with pragmatism, but under it all they want to rip to shreds whatever offends them. They just understand that if they destroy everything, they'll have no allies, no resources, and not much chance of achieving their aims.
Also not sure they would leave a Light side Sith alone, the majority are Dark siders who view the Light side as either weakness or treason, which makes a Light side Sith story dubious.
My DS warrior doesn't view the destruction of the weak as a cruelty, nor is she overtly mean as she goes about destroying them. She sees no reason or purpose for that, anyway. To her, she's merely doing what needs to be done, like taking out the trash, perhaps.
Would I call her a nice lady I'd like to hang out with, ever? Heck no. But to say that to be DS she must be a twisted, nasty, obscenely cruel and insanely mean psychopathic witch would, likewise, be untrue. Define your character as you choose, mind you. But my characters act according to their own moral codes, their definitions of what constitutes right and wrong, though. I try to approach every situation in the game more like that, than to follow some strict definition of "being bad means I pick the red button".