According to the Encyclopaedia Britannica, though Thomas Jefferson used to be considered a paragon of liberty, he has become increasingly criticized by scholars in recent years because of his beliefs on black people and the institution of slavery. Because of these views, he was a bad president because it was impossible for him to uphold the ideals of the Declaration of Independence, which he himself drafted.
Though in the Declaration of Independence Jefferson wrote "that all men are created equal, that they are endowed by their Creator with certain unalienable rights," he believed that there were biological differences between whites and blacks that made blacks inferior beings. Throughout his life he kept slaves on his plantation, sometimes around 200 at a time and possibly up to 600 in his lifetime. Though he claimed to have an aversion to slavery, he believed that blacks and whites were incapable of living together in harmony. His justification for continuing slavery was that it would be logistically difficult to transport all freed blacks back to Africa or to the Caribbean.
Thomas Jefferson almost certainly had an affair with a slave of his named Sally Hemings, and had several children with her, according to Biography.com. In 1802, a journalist first made this accusation. In 1873, one of Hemings' children corroborated the story. In 1998, further proof was found after DNA testing of ancestors of both Jefferson and Sally Hemings. Critics and admirers of Jefferson differ in opinion as to whether the relationship was forced or consensual.