I once had a bad reaction to a (prescribed) drug: it made me angry all the time. It's one of the experiences that made me realize that free will is not as free or willful as most people think. But it also shaped how I view orcs these days. Orcs were created by their gods to be warriors, they are innately ferocious. When you're angry all the time, it's easy to be chaotic evil. Even if an orc chooses to be lawful or good, they still have to either subsume or refocus that anger. (I suppose one might compare it to the many versions of the Incredible Hulk.)
In the same way humans are the baseline for ability scores, I think of humans as the baseline for alignment. We have both good and evil, lawful and chaotic instincts. Fantasy races with alignment tendencies diverge from "human nature" in meaningful ways. Dwarves don't have as strong a desire for individuality (thus a lawful alignment and a Charisma penalty). Elves live long enough that permanence is less important to them (thus a tendency to chaos). Whether good or evil, by humans standards, goblins never stop being precocious children (and thus devoid of other influences go Lord of the Flies).
Well said David, you have expertly surmised how I view fantasy settings. I think the core of the discussion falls to the age old nature vs nurture debate, and I would say for D&D its generally about 25% nature, 75% nurture.