If Lucas chooses not to make a black actor a bad guy because of his colour than that is rascist. If he did that then he'd be saying "you can't be a bad guy because your black and people won't like it". That sucks.
But I think if their is a traitor then it has to be Mace. He was the only Jedi Council member developed in TPM besides Yoda and we know it's not him.
why would ob1 be a sith, how did he turn back to the good side, the traitor in the jc might not be connected with sidious, wouldn't Yoda sense this traitor, secondly why would sidious run the risk of exposing himself to the jc, i suspect the traitor is linked with dooku or the nemoidians and the jedi thinks that when the jc falls he will rise to power.