Slavery was widely condemned by civilized nations in Jefferson's lifetime, whether or not Bill is too ignorant to know it.
Indeed, the civilized nation of England had taken the first step towards abolishing slavery as early as 1772, in Somersett's case. Somersett was a slave, owned by Charles Steward, an English colonial bureaucrat stationed in Virginia. One year, Steward travels to London with Somersett, Somerset runs away and joins a community of free Blacks in London. Steward has him caught and put on a ship to the Caribbeans.
Then English abolitionists get wind of the whole thing. They file a habeas corpus
petition and bring Somersett's case before the Chief Justice of the King's bench, Lord Mansfield. After much deliberation, Mansfield decides that slavery cannot exist under the Common Law. Only a statute enacted by parliament could establish slavery in England, and the House of Parliament has never done that. In other words, Somersett is free. And in the course of freeing him, the King's bench decides that slavery is immoral.
Four years later, many of the North American colonies declare independence, with slave owners from Virginia featuring prominently among the rebellion's leaders. Coincidence? I don't know --- post hoc ergo proper hoc
fallacy and all that. But it does make the whole freedom rhetoric of certain Virginian slave owners ring a bit hollow. What were their names again? I forget.