I know it's rarely the way people are, and in the Star wars world, most of the people aren't that way either. However, Jedi's and Siths are that way. Everyone else in the entire Star wars universe is more complex then black and white. Han Solo is more complex then simply White, as we know he has both at certain times. The senate, i'm sure has many good people, and many evil people on it. Corruption is abound on the senate also. It is not a black and white issue.
But when it comes to the religion of Jedi and Sith, it is that simple i'm sorry to say. It was specifically written that way. Jedi's are the embodyment of pure goodness. If you are not that way, then you are NOT a jedi. Anyone in the entire star wars universe who isn't purely good, isn't a Jedi. Jedi's must be by definition good. If there was a star wars dictionary, and you were to look up the word Jedi, it would say good. If you were to look up the word Sith, it would say evil. That is the inescapable truth about the story. Jedi=good, Sith=evil