Being dark side doesn't mean you're absolutely nasty cruel and terrible mean, even. You just kill the people who need to be killed, after all.
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.