Err I wouldn’t say that Persians were depicted as a bunch of “evil hellions” just power hungry.  What Xerxes did or was doing was attempting to control the world, no different than what Alexander the Great was doing when he whooped Darius the 1st who incidentally happened to be Xerxes’s father. Since I minored in both Greek and Roman mythology the battle of Thermopylae was one of the key points on my curriculum and I can state with confidence that, that battle was not a folk story made up by just any Greek, it was In fact a short war documented by the Father of History himself Herodotus of Halicarnassus.  He just happened to be the fist historian ever to accurately document and test his theories before relaying them to the masses.  The Battle of Thermopylae was just the precursor for an even larger battle, the Battle of Plataea which bought an end to the Greco-Persian wars.  In regards to the comment about Frank Miller 300 is the only comic book he ever wrote that depicted actual historical events and he only wrote 5 of them.  In my eyes all he did was make the Persian army a lot larger and more deadlier, formidable than what they really were at that time. His other writings included the Sin City stories, some Batman ( when they introduced the Dark Horse series) and Robocop.  Anywho I loved the movie but it was less for the battle and more for the well oiled men running around in their undies...
Cheers...