Sumner knew a lot about the character of slavemasters and their supporters. He cited Thomas Jefferson, who was of course a slavemaster himself, who also knew a lot about the topic:
"There must be an unhappy influence on the manners of our people, produced by the existence of Slavery among us. The whole commerce between master and slave is a perpetual exercise of the most boisterous passions, THE MOST UNREMITTING DESPOTISM on the one part, and degrading submissions us on the other; our children see this, and learn to imitate it. ... The man must be a prodigy who can retain his manners and morals undepraved by such circumstances. And with what execration should the statesman be loaded, who, permitting one-half the citizens thus to trample on the rights of the other, transforms those into despots, and these into enemies, destroys the morals of the one part, and the amor patrie of the other! ... With the morals of the people, their industry is also destroyed."The New York Times has the text of the speech online. I rely here on the text from the version published in 1863 as Barbarism of Slavery.