My sith warrior kind of ended up light side by accident. While I think the sith really should dial back the social darwinism, I can understand the importance that they place on bettering oneself through conflict and adversity, and not letting evolution and progress grind to a halt. By extension, my warrior sees the dark side as a necessary evil that must be included in the grand scheme of things, even if it has to be forced on the galaxy-- but moderation is key, and the empire, generally speaking, lacks moderation.
So imagine his confusion when he catches up to Jaessa and she tells him that his actions reflect only light, apparently due to his habit of doing as much of the empire's dirty work as possible so that others won't have to... oh, and being relatively merciful when he can. Who would have thought ethics could effect your alignment more that religion?
I have no idea if I'm explaining this in a way that makes any sense, so to illustrate: there was this one quest line on Taris where you figure out what to do about some nekghouls. The light side option is to form an alliance with them, and the dark side option is to kill them all. They deserve to live just like anything else, but the plague they carry must be contained at any cost.. I was pretty torn about it, and I'd imagine my sith would be, too.