I would figure that the idea behind "light side" Sith is to kill only when you have to, not when you feel like it, and to spare people who might potentially be useful to you. If you give them their lives, they may well give you their service. Something like that.
Honestly...I plan to experiment. My first character - an Inquisitor-Sorcerer - will be a complete sadistic asshole, heh. My next - I dunno if I will end up going Warrior, or Inquisitor-Assassin, just to test it (seeing as I don't think you can dual-spec) - will probably be a little more...thoughtful about the people he kills, heh.