We have the potential, each of us individually and in our species and society as a whole, to do great good as well as great evil. Anyone can be a slave or a master if we allow the liberal values and civilized trappings we’ve painstakingly built up over human history to slip back just a little. It takes effort and slow, steady social improvement to overcome our worst natures. [And there’s] nothing natural or inevitable about one ethnic group being on top and another being on the bottom, as even a cursory study of the history of nations makes clear.
For a taste of Heinlein’s dark side, read on for a little about Farnham’s Freehold.