In the late 1700's, blacks were inferior to whites. People believed that slavery was better than freedom for blacks due to their brutish nature. The kind white masters could civilize them, see?
By today's standards, all of those statements are blatantly racist. In their own time, they were mainstream belief.
You're criticizing men who (as a group, if not individually) were more enlightened than the society they lived in because they haven't had the 200 years of research and knowledge that you do.
Have you never read 'Frankenstein,' 'Treasure Island,' 'Little Black Sambo,' or other books from that era?