That was...not a founding pillar of the United States.

It's fairly well documented that the Founding Fathers were pretty much all opposed to slavery, but saw it as a huge political shit sandwich that would jeopardize Federalization. The United States is one of the only unions on the planet in which every single member state (without exception) voluntarily ratified and agreed to join. Had the Founding Fathers pushed for abolition, the South would never have joined the Union, and slavery may not have been abolished in the 1860s.

In order to ensure Federalization, the Founders simply ignored the question of slavery to get the slave-states to join the Union, and in fact built systems in place to incentivize the abolition of slavery. The Three-Fifths Compromise was a good example of this: the South outnumbered the North (when counting the slaves), and the South wanted slaves to be counted as one whole person in the census so that they would have outsized representation in the apportionment of seats in the House of Representatives. This was obviously unfair since those slaves were unfree and basically would have given rich slaveholders multiple votes. The Three-Fifths Compromise was a proposal by the abolitionist North to 1) neuter the power of the South at the Federal level, and 2) worded in such a way ("non-free persons") that if a Southern slave state wanted more House seats at the Federal level, all they had to do was abolish slavery.

A really good way to understand how the Founders felt about slavery is to look at how those that literally advocated for starting a new nation based fundamentally on the institution of slavery (the Confederates) felt about the Founding Fathers. The Cornerstone Speech was an address given by the Vice President of the CSA just before the Civil War began, and includes commentary around how they felt about the “old Constitution” and its framers: https://en.wikipedia.org/wiki/Cornerstone_Speech#Cornerstone

The relevant bit:

“The new constitution has put at rest, forever, all the agitating questions relating to our peculiar institution, African slavery as it exists amongst us – the proper status of the negro in our form of civilization. This was the immediate cause of the late rupture and present revolution. Jefferson in his forecast, had anticipated this, as the “rock upon which the old Union would split.” He was right. What was conjecture with him, is now a realized fact. But whether he fully comprehended the great truth upon which that rock stood and stands, may be doubted. The prevailing ideas entertained by him and most of the leading statesmen at the time of the formation of the old constitution, were that the enslavement of the African was in violation of the laws of nature; that it was wrong in principle, socially, morally, and politically. It was an evil they knew not well how to deal with, but the general opinion of the men of that day was that, somehow or other in the order of Providence, the institution would be evanescent and pass away. This idea, though not incorporated in the constitution, was the prevailing idea at that time. The constitution, it is true, secured every essential guarantee to the institution while it should last, and hence no argument can be justly urged against the constitutional guarantees thus secured, because of the common sentiment of the day. Those ideas, however, were fundamentally wrong. They rested upon the assumption of the equality of races. This was an error. It was a sandy foundation, and the government built upon it fell when the “storm came and the wind blew.””