It's not going to be humans ARE evil, rather that whether evil or good humans are unique in that they are the rare combination of capabilities and will to impose - lowly Orcs may have the will and desire to control others, but lack the ingenuity and creativity of humans to grow in strength. Thus, the differences between, say LG and LE are not as far apart as one might think - both seek to impose their will on others, but to different ends.
My thought is that humanoids, creatures, etc. existed alongside demi-humans before humans arrived, but that a natural balance existed (chromatic/metallic dragons) that was disrupted by the arrival of humans. Thus, it's not the "creation" of races, but the evolution of their cultures that reacts to the imbalances in the world created by humans - they build empires, exploit multiple and extensive portions of the lands, etc.
The villains I am employing are to be exemplars of this. In some respect I am influenced by Sith mythology from Star Wars (don't worry - no tech or sci fi, just the philosophical aspects) and am creating an ancient lineage of Dark Lords, whose evil is a philosophical choice - that man has the will to dominate or not, and they simply accept and choose this.
Ecological overtones most certainly, but I think the colonial comparison is perhaps the best. Noble intentions of LG to "civilize" or "bring the light" to others (i.e. heathens or savages) attempts to displace or overwrite the culture of others.
So following up on the thumbnail - I think the main purposes of heroes is to engage adventures that will reveal the history and nature of man in these Lands, and (following on the Players should have an impact) give the chance to possibly shift this imbalance.
Thanks again - your comments really are very helpful!
John Maddog Wright
"No matter where you go, there you are... in a tavern!"