I'm not sure how you would teach about slavery from a neutral perspective (i.e. everyone else was doing it, so it was OK). You don't have to teach that slavery was bad. Just teaching the facts about it would lead most students (if not all) to the conclusion it was bad. If I am teaching about slavery in the 1800s just prior to the Civil War, would I be expected to interject some unrelated information about slavery in Africa to give the appearance of "balance?"