I think you could make the same argument that in the cultural context of the time, many felt the Spanish Inquisition was justified and was truly helping people get to heaven. Colonialism and exploitation of natives probably made perfect sense to the Spanish coming to the Americas, even if now in today's context it's obvious they were pretty brutal and cruel. 
Of course you could make that same argument, but with the benefit of history and progress, we have the luxury of looking back and acknowledging that things like human sacrifice and the spanish inquisition (regardless of the underlying rationales) were "weird and violent and gruesome" -- which are labels she specifically disputes as being "right." Smart people rightfully label slavery as evil and horrific despite whatever moral/theological defenses southern intellectuals were putting forth during antebellum.
I think a lot of people understand that there is some implicit theological underpinning to "human sacrifice," but that shouldn't absolve the practice from being labeled "horrific."
Yeah I still think context matters. IMO "violent and gruesome" are fair but I don't think "evil and weird and horrible" are when you attempt to look at it through the eyes of an Aztec in the 15th century, because it was the standard in the society they knew.
As for the Antebellum South, they had plenty of people all over the world telling them they were "evil" and they were explicitly trying to justify their actions. As far as I know, the Aztecs had no idea what they were doing was "evil" or "weird" which makes it difficult to apply those labels to them in context.
A sorta similar conversation could be had surrounding our culture's revulsion to eating dogs while we eat bacon like crazy without remorse. How can one be "weird" or "evil" and the other not be without considering the context of our present-day culture?
