I honestly have never known anyone to deny that they have a dark side, but then again, I've never really known anyone to actually come forward and proclaim they have a dark side... everyone has a dark side, I guess some people's dark sides are more dominent in their personality than other sides, and as such, are more vocal about it than others.
Sure, I have a dark side myself, but it doesn't show itself too often, so I don't think about it a whole lot, so I don't go around talking about having a dark side.