When shagging the dragon, don't forget your rubber
Starting with Beowulf, a heavily adapted version of the old Anglo-Saxon poem by the same name, the movie describes how a great hero arrives at a Scandinavian village that is being attacked by a creature that is (unknown to the population) a bastard child between its king and a local dragon/demon. After slaying the poor creature, our hero then decides to shag said dragon while lusting for the king's wife. Needless to say, another bastard half-monster comes out of this escapade and later haunts the same village. If you have not caught the hidden meaning by now, I will tell you, the problem is the fucking promiscuity of some rulers, and their careless captain Kirk-style sexual appetite resulting in at best a venereal disease or at worst a burned down village.
Now regarding 300, I think it is needless to say what a load of elephant crap the whole movie is, and how the "documentaries" that accompany the blue-ray version of it only make it worse at trying to justify their adaptations for dramatic license (my ass, I tell you). But I will summarize the bull crap part: the movie essentially tries to transmute a culture of misogynistic pederast autocrats with less than a third of the population having voting rights into a tale of heroics, and I shit you not, freedom. They even get the classic "freedom is not free" shit that some Americans are repeating ad nauseam to justify their current policies. But digressions apart, the movie portraits what was probably one of the greatest empires of the time, known for its religious freedom and prohibition of slavery into a Lord of the Rings style evil horde. They also overlook the fact that that particular war was won by the military cunning of Athens rather than the Hooah-style mindless charge that is typical of some modern military units.
Little Quasimodo just wanted his chance
The movie seems to glorify their intolerance to any people that is not conforming with the standards of the tribe, so when our Quasimodo lookalike tries to help his own nation, he is bluntly dismissed as unfit, because he cannot raise his shield to a certain height, even considering that most of the formations portrayed in the movie have leaning men, and they do not last 10 seconds of movie together. And here comes our second hidden meaning, the Persians in this movie, despite attempts to turn them into monstrosities, clearly embrace diversity, and their army has people from all over the world. This empire accepts Ephialtes into their ranks, and that proves instrumental to their victory. So, moral of the story is, embrace diversity and win.
Coincidentally, both movies have been adapted from or by writers of comic books, so we might have a pattern here. In all honesty, Beowulf is a vastly superior movie, which is pretty compatible with Neil Gaiman's work, whereas Frank Miller's work is mostly a bloodbath, aiming to please your average frat boy demographics (though I like his Batman stories).