Sen. Charles Sumner in his famous Barbarism of Slavery speech speech of 1860 from which I've been quoting in several posts, also noted a couple of examples in addition to Thomas Jefferson who criticized the institution of slavery. One was John Locke:
Next comes the Philosophic Authority; and here the language which I quote may be less familiar, but it is hardly less commanding. Among names of such weight, I shall not discriminate, but shall simply follow the order of time in which they appeared. First is John Locke, the great author of the English system of Intellectual Philosophy, who, though once unhappily conceding indulgence to American Slavery, in another place describes it, in words which every slave master should know, as--
"The state of war continued between a lawful conqueror and his captive. ... So opposite to the generous temper and courage of our nation, that 'tis hardly to be conceived that an Englishman, MUCH LESS A GENTLEMAN, should plead for it."Then comes Adam Smith, the founder of the science of Political Economy, who, in his work on Morals, thus utters himself:
"There is not a negro from the coast of Africa who does not possess a degree of magnanimity which the soul of his sordid master is too often scarce capable of conceiving. Fortune never exerted more cruelly her empire over mankind, than when she subjected these nations of heroes to the refuse of jails of Europe, to wretches who possess the virtues neither of the countries which they came from, nor of those which they go to, and whose levity, brutality and baseness so justly expose them to the contempt of the vanquished." -- Theory of Moral Sentiments, part 2. Chapter 2.This judgment, pronounced just a country ago, was repelled by the Slave-masters of Virginia, in a feeble publication which attests at least their own consciousness that they were the criminals arraigned by the distinguished philosopher. This was soon followed by the testimony of the great English moralist, Dr. Johnson, who, in a letter to a friend, thus shows his opinion of Slave-masters:
"To omit for a year, or for a day, the most efficacious method of advancing Christianity, in compliance with any purposes that terminate on this side the grave, is a crime of which I know not that the world has had an example except in the practice of the planters of America, a race of mortals whom I suppose, no other man wishes to resemble." -- Letter to William Drummond, 13th August , 1766. (Boswell's Life of Johnson, by Corker.)
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.