History students are typically taught to understand heinous behaviors within the context of the times in which they were practiced. The enslavement of Africans in the United States is one example of a barbaric past practice to which people have applied this rule, disregarding the overwhelming evidence that it could not even have been considered moral when the fledgling republic first won its independence from Great Britain.

Slavery was already a contentious issue in the West as early as the Revolutionary War. In 1777, only a year after the United States declared its independence, Vermont became the first state to abolish slavery. In 1776 and 1778 respectively, Delaware and Virginia banned the importation of slaves. Perhaps most significantly, the Constitution declared a nationwide ban on importing slaves in 1808, and doing so would become punishable by death only 11 years after. 

Despite owning slaves, Thomas Jefferson had moral reservations about the practice

While it is often believed that only abolitionists opposed slavery in the United States, even some slave owners had moral reservations about the practice. Thomas Jefferson epitomized the ashamed American slaveholder. As the U.S. Minister to France, he had one of his slaves, a woman named Sally Hemings, live with him in Paris as a servant from 1787 to 1789. Because France had already abolished slavery by that point, Jefferson downplayed her background as one of his slaves and emphasized her role as his mistress, according to Ibrahim Sundiata, a Professor Emeritus of History at Brandeis University. 

As evidenced by the early efforts of many states to abolish slavery and/or ban its importation, along with the guilt of many slave owners, such as Thomas Jefferson, over their "property," it is impossible to thoroughly moralize slavery in American history.