Edit: Why shouldn't Sith be able to do a good deed? If the Dark Jedi can flay a person alive while showing no emotion(quite psychopathic), why shouldn't Light Sith be Martyrs or whatever of the Empire?
Light leaning Sith are mercilessly hunted in the Empire. What few exist either conceal their true nature from others or die. Not that a light Sith couldn't do good deeds, but there's a limit to how far they can go with being noticed.