Actually, Darth Caedus (Jacen Solo, son of Han Solo and Leia Organa Solo) becomes a sith to save the galaxy from Corellia's threat bla bla bla. He becomes a sith because Lady Lumiya convinces him that being a sith doesn't mean being necessarily evil, because a sith lord before her, (I forget his name I'm sorry) was a good hearted sith but did what had to be done for the good of the galaxy and the sith. He had a family, he wasn't psychotic, he didn't kill (at least not ruthlessly). He wanted the sith to rule the empire, but he always refused to be in any way evil about it. Caedus bases his values off that. Lumiya was evil but she's more like grey in the books, she doesn't seem evil in any way and she kicks ***, brings Jacen to the dark side and teaches him about being a sith.
TL;DR AND DIDN'T CARE: There we're some succesful light sided sith in the STAR WARS lore already, and they were succesful.