(Note: I am neither American nor German, and all my problems are definitely of the first-world variety. I hope to be thoughtful, but I am sure to be ignorant.)

In the one case, millions of people (say 5% of the country?) were terrorized, then kidnapped from their homes, then treated astonishingly badly, and then many of them were murdered. And it happened during a brief period of 5 years or so, 60 years ago. Short duration, but pretty recent.

In the other case, first of all, consider only the so-called Middle Passage. The total number of people involved is roughly comparable (over 10M), and mortality rate was perhaps a little lower, but roughly comparable (millions died). There then followed generations of maltreatment; appalling as the physical treatment is, the long-term cultural impact of generations of forced ignorance is arguably worse. (I think we can all agree that the oppressed people of Germany are doing much better now [65 years after 1945] than were, say, the Americans of African descent in 1930 [that's 65 years after 1865].) The only thing that seems less horrifying about the era of slavery is that it's a little more distant, at least the really brain-wreckingly horrifying parts.

So, in conclusion, it's not at all clear to this outsider why references to "death marches" are "way worse" than references to "niggers". Seems to me like they're pretty comparable.