Which of the founders was a true humanitarian. I've looked up which ones didn't own slaves and couldn't find any.
Among the famous founders, just Thomas Paine, Samuel Huntington, and the Adams family. Also of course plenty of obscure figures -- Wikipedia says fifteen signers of the DoI didn't own any slaves.
Samuel Adams would be a better example, as he was a life-long abolitionist and used his influence as a business leader to discourage slavery in his nascent state.
The exception that proves the rule. Sam Adams received a slave as a gift. He freed her immediately.
This is moral relatavism nonsense. People hundreds, indeed thousands of years, ago knew slavery was wrong. It is why Spartacus revolted.
This is anachronism nonsense. Spartacus revolted because being a slave sucks. There's no record of Spartacus raising any objection to the enslavement of anyone besides himself and his own followers. Not wanting something for yourself in no way means you don't want it for others -- you might as well argue that every soldier who tries not to get killed knows war is wrong. The normal attitude toward slavery in ancient times was that it was a personal misfortune. There was no abolitionist movement; nobody even spoke out against it but a few scattered weirdos. The Persian Empire had laws against it, true; but they only prohibited enslaving Zoroastrians.
These things were always known to be immoral.
Maybe not by all of the perpetrators, but certainly by all of the victims.
There's a long list of former slaves who became slaveowners, in ancient times, in antebellum America, and elsewhere. Cicero famously observed "A slave dreams not of freedom, but of his own slaves." No doubt some of these were as hypocritical as Thomas Jefferson; but others undoubtedly absorbed the popular attitude of their times that slavery was fine as a practice and it was only sensible to want to be on the dishing out end rather than on the receiving end.
To my mind, the true test of whether some dead guys knew it was wrong is whether they lied about the reasons for it. The ancient Spartans ritually declared war on their slaves every year for hundreds of years after the original war, to perpetuate the fiction that the practice was still about war and not exploitation. During the Atlantic slave trade era, slaves were treated better or worse in the receiving countries according to how expensive or cheap they were to replace by kidnapping more Africans; that would not have been the case if any of their proffered justifications were their real reasons. So yeah, most of those perpetrators have to have known it was wrong.
But the more important point is that all the perpetrators
should have known it was wrong.
