Again, I disagree with the assessment that the Force is simultaneously good and bad: I think the Dark Side of the Force is a corruption of what it ought to be. Who decides what it ought to be? If the force has a will, it indeed allows the Sith to use it's dark side. Remember, it is the Dark Side of the Force who control the Sith's destiny. Not the other way around. Corruption or not, it's a natural part of the force, not something the Sith made up. In fact, the dark side was already present in Anakin much before any Sith Lord even tried to train him. Yoda talks of the dark side on TPM as a natural consequence of human emotions, not as a corrption created by men. EDIT: Added a comment.