Jefferson's relationship with slavery was a complicated one. Yes, he owned many slaves at his Virginian plantation, but the man who wrote "all men are created equal," genuinely supported the abolition of the institution. In 1778, he even wrote a law in Virginia that banned his home state from importing more slaves.
John Boles, a professor of history at Rice University who has written about Jefferson, commented:
He was sincerely against slavery. He just didn't know how he could end it for himself or for the nation, but he believed slavery was terribly wrong, and he spoke and wrote against it many times.
Jefferson was born into a slave society and inherited his slaves, some of which he did eventually free. Annette Gordon-Reed, a professor of history at Harvard who wrote a book about Jefferson and his slaves ("The Hemingses of Monticello: An American Family") had this to say on the subject:
Jefferson certainly believed slavery was an evil. He lost faith in his fellow Virginians' desire to do anything about the institution and concentrated on other things, namely the politics of United States of America.
Though there's more to be said about Jefferson's actions, and how they sometimes didn't align with his noble beliefs, but there's no doubt that his words would have made quite the impact if left in the Declaration of Independence. Gordon-Reed commented:
He has some pretty harsh words to say about the slave trade and he recognizes Africans as human beings in a very eloquent way. I think those words could've been used throughout American history had they remained in the Declaration.
Sadly, America wouldn't be ready for Jefferson's idea for nearly a hundred years.