Star Wars Interlude: Is Luke Light or Dark

The light side of the force is the side of logic and reason. The dark side is the side that is fueled by emotion and passion. Neither is good or evil. They're merely different paths to walk. In fact, to give in to either side completely--be that to fall entirely within the sway of your emotions without reason to guide you, or to become a creature driven entirely by cold logic without compassion to make you merciful--can be evil. In fact, I have quite a long rant about Vader on this subject.

The force, as a complete power, yearns towards balance. In the sense that what is natural and right can be called "good," balance must be good. Not light or dark.

Let's start with the nature of the force: It is repeatedly stated that being controlled by your emotions is the primary indicator of the dark side. And not just those emotions which we deem negative. Anakin weds Padme in secret because love is viewed as dangerous. By contrast, the light side is the side of calm, rational decision making. This isn't conjecture on my part, this is accepted cannon. By defining the two sides of the force via the code of the Sith and the Jedi, you're putting the cart before the horse. Their codes don't define the force... the different aspects of the force shaped the users and their code.
So the question isn't "what defines the light and dark side of the force?" The question is, "is either side inherently evil?" And the answer is pretty blatantly no, despite the constant confusion of the general populace between the dark side and evil.
 
After all, Vader's greatest act of redemption is done out of love: and love is an emotion. It is, by definition, part of the dark side of the force. But it was also an act we can all universally agree as good: a father acting in defense of his son. 
 
Don't forget, the idea that the Sith are evil come from the Jedi, and the Jedi HUNTED AND EXTERMINATED as many Sith as they could find, because they had differing religious beliefs. Out of cold, merciless logic. That doesn't make it "good". But it was totally light side, all the way.
Emotions and logic are merely tools we use in decision making. Good and evil are value judgements we place on any given decision. The tool used in making the decision does not inherently influence the value of the decision one way or the other.

Now as to the question of whether Luke went dark.
 
I actually don't think Luke is a Sith. Sith implies that he believes their dogma that the dark side is "better". I also don't think he's Jedi, as that implies that he believes their dogma that that the light side is "better." I think Luke--as one of the few remaining force wielders left after the Jedi killed the Sith and then the Sith repaid the favor by killing the Jedi--has come to realize that neither side is inherently good. That logic without love can cause just as many problems as emotion without reason. And therefore I think we will see a Luke who strives for balance within the force, and in order to maintain that balance, yes, he will use the dark side on occasion.