people are often saying that the tolkien books are all "black and white". there are evil guys and there are good guys. like starwars. but I wouldn't say that. not even morgoth is described a "bad guy". he just sang his own tune that's all. fëanor was fooled by morgoth, not evil. so even if the men of harad are black and servants of sauron that does not make them evil. ther're just on the wrong side.
another thing that would prove that the books don't have anything to do with Nazism is that the good side eventually wins. if sauron was supposed to be Hitler, then why did he loose?