he has waged cruel war against human nature itself, violating it's most sacred rights of life & liberty in the persons of a distant people who never offended him, captivating & carrying them into slavery in another hemisphere, or to incur miserable death in their transportation thither. this piratical warfare, the opprobrium of infidel powers, is the warfare of the CHRISTIAN king of Great Britain. determined to keep open a market where MEN should be bought & sold, he has prostituted his negative for suppressing every legislative attempt to prohibit or to restrain this execrable commerce; and that this assemblage of horrors might want no fact of distinguished die, he is now exciting those very people to rise in arms among us, and to purchase that liberty of which he has deprived them, & murdering the people upon whom he also obtruded them; thus paying off former crimes committed against the liberties of one people, with crimes which he urges them to commit against the lives of another.
See Jefferson's full "rough draft".
This paragraph was included by Jefferson in reaction to the king's disallowing laws passed by the Virginia legislature that would have interfered with the transatlantic slave trade. There were at this time many in Virginia, including many slave owners like Jefferson, that felt that slavery was wrong; it was at best a "necessary evil". Virginia at this time had more slaves than any other colony. However, slavery in Virginia was becoming less of an economic necessity, and Virginians were beginning to sell their surplus slaves in other colonies farther south. This trade became increasingly important when the cotton culture dominated the south. Abolition of the transatlantic slave traffic would make Virginia slaves more valuable.
Jefferson did propose that the United States begin, in 1800, gradual abolition of slavery. His proposal was narrowly defeated in Congress. However, his proposal to ban slavery in the Northwest Territory (in the Northwest Ordinance of 1787) was adopted.
See the original draft, the adopted draft, and the engrossed version.