I haven't watched this movie since it came out in 2006, I was 18 at the time.
It was obviously propaganda from the mouthpiece of the Spartan telling the story. I had always thought that he was just making the Persians seem like monsters because that's who they were up against. I think that's another reason that Xerxes was like 10 foot tall and chiseled, because he was trying to emphasize what type of evil they were up against. Reading some points in here kind of make me want to watch the movie again, under a new set of eyes/values. Granted at 18 I wasn't ask aware of politics, social issues, etc as I am now, so maybe that had something to do with it.
Interesting read.