I think Mace could end up being a dark jedi when all is said and done. Let us not forget the cave in Empire where the darkside is strong, and how Yoda reacts to the cave, not wanting to look in the direction of the cave as well as an almost sad look on his face. Why would some random place on dagobah be so strong with the darkside? I think yoda defeats a dark jedi here and the aura of that dark jedi is left in the cave. Whether it is Mace or Dooku, who knows? But i think it could be mace, which would explain why yoda seems so sad near it, remembering howhe killed an old friend, maybe?