Northup is saying that not all slave-owners are bad. Most are a product of their culture and society. They succumb to peer pressure, they do what they are supposed to do because that is how everyone else expects them to behave. William Ford treats his slaves because of peer pressure.
Peer pressure is often a determining factor in why some people react the way they do. People hate being ostracized, because it’s human nature to be social. In high school, if everyone is an athlete, everyone wants to be an athlete, because that is how you interact with your peers and classmates. They have all the cool stuff, but at the same time, get shut out enough, and someone stops trying to interact with others, and that is when people develop in a divergent path.