The story of few good, brave, strong men vs. countless evil beings is not a new concept in this world. It just so happens that the "bad" side actually existed as the Persian Empire. I really do not think Miller's decision to make the story like that and to put the Persians in such a light is that big of a crime against humanity. He was just trying to stress the valor of the Spartans and make them appear in an even better light. Any person with common sense knows the Persians didn't have an army of warlocks or creatures from the pits of hell. And don't forget, part of the Spartan culture was portrayed as really harsh (ie the evil priests and the tossing of babies off of cliffs). The movie certain stretches things (and in the case of the Persians, quite a bit), but it is also very enjoyable, and above all is just a story.
"There are many dying children out there whose last wish is to meet me" -David Hasselhoff