I must say it is important to talk about slavery because during the last century many historians would barely mention the horrors of the institution. Having said that, one should not use presentism when reviewing past events. Thomas Jefferson was a slave owner and a man of his time. He also was an exceptional man of his time, he was a great intellectual, writer, religious freedom advocate, and yes an abolitionist. If we are to tell true history, then we must teach the good the bad and the ugly.
I understand that every historian or tour guide is biased by the mere fact they are human, but we must strive to be balanced and tell the truth even though it goes against a political narrative.