I would agree but....
1) He went above and beyond belittling them. That almost ruined the mission (as they were on the horses coming in to Candyland, right before the incident with the fighter and the dogs). Am I forgetting something? I get that he was trying to show how he was a black man doing a horrible job but what he did was unneccasary. Did this help the mission at all?
2) it would have cost him nothing to say something/assist those also on his way to the mines (after he blew up Quentin)
3) In one scene he is saying that a black Mandingo trader was a horrible person, but in the next scene he is being the worst kind of Mandingo trader. In my opinion, that means he was aware and had the capacity to care for another slave.
The dog scene bothered the doc, but Django, nope.
In the scene on the way to the mine, I think I just wanted Django to seem as smart as the doc was in the first scene. The doc was smart. The doc cared. The doc grew. Why not Django (in ways other those that were violent)?
Even the smallest gesture, like tossing them keys or saying "head in that direction"