As reluctant as I am, I have to agree with everyone else here that darkness is not always evil. In fact, it is neither good nor evil. The same goes for light. Darkness is just another thing that, in the case of KH, can be weilded as a weapon against enemies. The reason why the stereotypical person (well, not nowadays anymore) automatically links darkness to evil probably comes from some sort of survival instincts. After all, humans don't see very well in the dark while predators who may hunt us down may see us as plain as day.
But then again, I have no analysis background and absolutely suck at history. You have my total permission to disregard everything but the first sentence. ;p